(CNN) Nearly two-thirds of the US Navy's F/A 18 strike fighter jets are currently unable to fly, grounded due to repair delays or because they are awaiting spare parts.

With more than half of all Navy aircraft out of service and no budget agreement in place to increase defense spending levels, the top brass says its usable planes are being pushed to the limit.

"For a variety of reasons, our shipyards and aviation depots are struggling to get our ships and airplanes through maintenance periods on time," Vice Chief of Naval Operations Adm. William Moran told lawmakers on the House Armed Services Committee earlier this week.

The number of non-operational F/A 18 Hornets is "double where we should be," he said, confirming reports that 62% of the Navy's F/A 18s are unable to fly and 53% of the Navy's total air fleet is grounded.

Moran, joined by commanders from the Army and Air Force, described an aging, overworked and undermanned air fleet that is feeling the weight of sequestration and furlough budget cuts implemented by Congress and the Obama administration.

But Peter Singer, a strategist for the Washington-based New America Foundation, said that "the readiness problems are a culmination of a series of decisions to keep kicking the can down the road, from Congress's budget issues to the Pentagon assuming there would be a new replacement jet by now."

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Democratic Rep. Adam Smith of Washington, however, cited budget caps implemented by Congress and President Barack Obama in 2011, coupled with the high demand for the fleet's services, as key factors contributing to the Navy's lack of fiscal flexibility.

"The budget caps imposed on military and nonmilitary spending in the Budget Control Act have degraded our military readiness," Smith, the ranking member on the House Armed Services Committee, told CNN, adding that those constraints have been "compounded by over a decade of war" and "long delays in the development of replacement aircraft."

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Congress opted not to address restrictions resulting from the Budget Control Act this year, failing to produce a budget before the October 1 start of fiscal 2017. Instead, lawmakers chose to use stop-gap measures that kept defense spending at the 2016 levels.

One Republican House Armed Services Committee aide told CNN that simply blaming Congress for the military's degraded readiness is an over-simplification of the issue and that today's problems are a result of a "disconnect between the previous administration and Congress" over matching budget cuts for domestic and defense programs under sequestration.

Photos: US aircraft carriers Photos: US aircraft carriers Sailors spell out #USA with the American flag on the flight deck of the Nimitz-class aircraft carrier USS Theodore Roosevelt in the Persian Gulf in late June 2015. When the Roosevelt leaves the Gulf sometime in October, the U.S. Navy will be without a carrier in the important region for two months. Hide Caption 1 of 31 Photos: US aircraft carriers A MV-22B Osprey, from Marine Operational Test and Evaluation Squadron 1, lifts off from the flight deck of the aircraft carrier USS Carl Vinson (CVN 70) on June 12, 2016. Click through the gallery to see other US aircraft carriers. Hide Caption 2 of 31 Photos: US aircraft carriers Tug boats maneuver the aircraft carrier Pre-Commissioning Unit Gerald R. Ford (CVN 78) into the James River during the ship's turn ship evolution on June 11, 2016. This is a major milestone that brings the country's newest aircraft carrier another step closer to delivery and commissioning later this year. Hide Caption 3 of 31 Photos: US aircraft carriers The Nimitz-class aircraft carrier USS Ronald Reagan (CVN 76) arrives at the U.S. Navy base in Yokosuka, a suburb of Tokyo, Japan, on October 1, 2015. The Reagan is the fifth U.S. carrier forward deployed to Japan following USS George Washington (CVN 73), USS Kitty Hawk (CV 63), USS Independence (CV 62) and USS Midway (CV 41), according to the Navy. Hide Caption 4 of 31 Photos: US aircraft carriers The aircraft carrier USS Dwight D. Eisenhower (CVN 69) (Ike) transits through the Strait of Gibraltar into the Mediterranean Sea on June 13, 2016. Ike, the flagship of the Eisenhower Carrier Strike Group, is conducting naval operations in the U.S. 6th Fleet area of operations. It could be used to support operations against ISIS in the Mideast. Hide Caption 5 of 31 Photos: US aircraft carriers A rainbow forms over the bow of the Nimitz-class aircraft carrier USS John C. Stennis as the ship steams in the Pacific Ocean on February 3, 2015. Hide Caption 6 of 31 Photos: US aircraft carriers U.S. aircraft carrier classes – U.S. Secretary of Defense Ash Carter (left) and Philippine Secretary of National Defense Voltaire Gazmin shake hands on a Marine Corps V-22 Osprey as they depart the the aircraft carrier USS John C. Stennis (CVN 74) after touring the aircraft carrier as it sailed in the South China Sea on April 15, 2016. Hide Caption 7 of 31 Photos: US aircraft carriers A photo illustration of the U.S. Navy's Gerald R. Ford-class aircraft carrier USS John F. Kennedy (CVN-79). The ship's keel laying ceremony was celebrated Saturday, August 22, 2015, in Newport News, Virginia. The ship is expected to replace the USS Nimitz (CVN-68), scheduled for inactivation in 2025, in the Navy fleet. The newest Kennedy will be the second carrier of that name. The first John F. Kennedy (CVA-67) was the last conventionally powered carrier. It was decommissioned in 2007. Hide Caption 8 of 31 Photos: US aircraft carriers The aircraft carrier USS Harry S. Truman (CVN 75) passes under the Friendship Bridge while transiting the Suez Canal on Dec. 14, 2015. The ship is conducting operations in the Persian Gulf, where Iran claims to have taken footage of the carrier using a drone. Click through the gallery for more images of U.S. Navy aircraft carriers. Hide Caption 9 of 31 Photos: US aircraft carriers Aviation Structural Mechanic Airman V. Sek, assigned to the "Jolly Rogers" of Strike Fighter Squadron (VFA) 103, applies a Christmas decal to an F/A-18F Super Hornet in the hangar bay of the Nimitz-class aircraft carrier USS Harry S. Truman in December 2015. Hide Caption 10 of 31 Photos: US aircraft carriers The Nimitz-class aircraft carrier USS George Washington (CVN 73) is seen from inside its sister ship, the USS Ronald Reagan (CVN 76), on August 7 off the coast of California as the two ships prepare for a "hull swap." Over 10 days in San Diego, much of the crew of each ship will transfer to the other. When completed, the Reagan will head to forward deployment in Japan, where the Washington had been. The Washington will head to Newport News, Virginia, for an overhaul. Hide Caption 11 of 31 Photos: US aircraft carriers Three Nimitz-class aircraft carriers USS Carl Vinson (CVN 70), top, USS Ronald Reagan (CVN 76), center, and USS John C. Stennis (CVN 74) are pierside at Naval Air Station North Island near San Diego on June 12, 2015. The Vinson has just recently returned from a 10-month deployment. The Reagan is preparing for a move to Japan later this year and the Stennis was making a port call after steaming from its homeport of Bremerton, Washington. Hide Caption 12 of 31 Photos: US aircraft carriers Sailors test the countermeasure washdown system on the flight deck of the Nimitz-class aircraft carrier USS Dwight D. Eisenhower (CVN 69) during sea trials prior to returning to its homeport at Naval Station Norfolk in late August 2015. Hide Caption 13 of 31 Photos: US aircraft carriers The aircraft carrier USS Theodore Roosevelt departs Naval Station Norfolk, Virginia, on Wednesday, March 11, for a scheduled deployment. The Nimitz-class carrier's departure was delayed for two days after marine growth clogged sea water intakes. Divers went into the 36-degree water to clean out the intakes and allow the ship to get under way. The cold water created a fog that made it seem the ship was in a cloud. Hide Caption 14 of 31 Photos: US aircraft carriers The USS Abraham Lincoln, a Nimitz-class aircraft carrier, is seen near the coast of Indonesia in 2005. The carrier recently received a new anchor from the decommissioned USS Enterprise. Hide Caption 15 of 31 Photos: US aircraft carriers Lightning strikes over the flight deck of the USS John C. Stennis, another Nimitz-class aircraft carrier, as the ship moves through the Persian Gulf in 2007. All of the Navy's 10 active aircraft carriers are from the Nimitz class, which started in 1975 with the commission of the USS Nimitz. Hide Caption 16 of 31 Photos: US aircraft carriers The USS Ranger (CV-61) arrives at Pearl Harbor, Hawaii, in 1993. The Forrestal-class carrier, which featured in the movie "Top Gun," is to be scrapped this year. Hide Caption 17 of 31 Photos: US aircraft carriers In this photo released by the U.S. Navy, a tugboat works alongside the decommissioned aircraft carrier USS Saratoga on Thursday, August 21, in Newport, Rhode Island. The Navy has paid a Texas recycling company a penny to dispose of the Saratoga, part of the Forrestal-class of "supercarrier" vessels built for the Atomic Age. The carrier was decommissioned 20 years ago. Hide Caption 18 of 31 Photos: US aircraft carriers Aircrew members are lifted from the flight deck of the USS John F. Kennedy during an exercise in 2002. The ship, which was decommissioned in 2007, was the only member of its class. Hide Caption 19 of 31 Photos: US aircraft carriers An F/A-18 Hornet launches from the USS Enterprise in 2007. The Enterprise, the world's first nuclear-powered aircraft carrier, was decommissioned in 2012. Like the John F. Kennedy, it was the only ship built in its class. Hide Caption 20 of 31 Photos: US aircraft carriers The Kitty Hawk class was named for the USS Kitty Hawk, seen here departing Yokosuka, Japan, in 2008. At that time, the Kitty Hawk was the oldest carrier in the U.S. Navy and the only conventional-power aircraft carrier still in commission. It was decommissioned in 2009. Hide Caption 21 of 31 Photos: US aircraft carriers The USS Independence, a member of the Forrestal class that preceded the Kitty Hawk class, heads up the East River in New York in 1959. Hide Caption 22 of 31 Photos: US aircraft carriers Helicopters sit on the flight deck of the USS Saipan during the mid-1950s. The ship was one of two members of the Saipan class. Hide Caption 23 of 31 Photos: US aircraft carriers The USS Midway, namesake of the Midway class of aircraft carriers, floats off the coast of North Vietnam in 1972. It was named after the Battle of Midway, when U.S. forces held back a Japanese attempt to take the Pacific atoll in 1942. Hide Caption 24 of 31 Photos: US aircraft carriers The USS Princeton, part of the Independence class, moves off the coast of Seattle in 1944. Hide Caption 25 of 31 Photos: US aircraft carriers The Essex-class USS Franklin burns after being hit by a Japanese dive bomber in 1945. The ship was named after Benjamin Franklin and nicknamed "Big Ben." Hide Caption 26 of 31 Photos: US aircraft carriers The USS Wasp burns in the Coral Sea after being struck by three torpedoes from a Japanese submarine in 1942. The ship, the only one of its class, would ultimately sink because of the damage. Hide Caption 27 of 31 Photos: US aircraft carriers B-25 bombers sit on the deck of the USS Hornet in the Pacific Ocean in 1942. The Hornet, one of three carriers in the Yorktown class, was the ship that launched the bombers flown by Air Force Lt. Col. James Doolittle and his pilots during an air raid in Tokyo four months after the attack on Pearl Harbor. It also was involved in the Battle of Midway. Hide Caption 28 of 31 Photos: US aircraft carriers Navy personnel work on board the USS Ranger circa 1942. The Ranger was the first ship to be designed and built specifically as an aircraft carrier. It was the only ship in its class. Hide Caption 29 of 31 Photos: US aircraft carriers There have actually been two aircraft carriers named after the Revolutionary War's Battle of Saratoga. The first USS Saratoga, seen here moving toward San Francisco's Golden Gate Bridge in 1945, was one of two members of the Lexington class of aircraft carriers. Hide Caption 30 of 31 Photos: US aircraft carriers The USS Langley, the Navy's first aircraft carrier and sole member of its class, steams off the coast of Baltimore in 1924. Hide Caption 31 of 31

Often described as the backbone of naval aviation, the F/A 18 was designed to have a lifespan of roughly 6,000 flight hours.

Today, jets are being stretched to fly between 8,000 and 9,000 hours to fulfill mission expectations as a result of fewer operational aircraft, budget restrictions and delays to the fifth-generation F-35 Joint Strike Fighter.

Moran warned that the shortage of flyable aircraft in an already aging fleet has left the Navy with "no depth on the bench" for future conflicts.

John Venable of the Heritage Foundation said the problems highlighted by Moran and other commanders illustrates the US military's "extraordinarily low levels of readiness" and that things could get worse before they get better.

Photos: In the U.S. Air Force fleet Photos: In the U.S. Air Force fleet B-52 Stratofortress – The first versions of this long-range heavy bomber flew in 1954. A total of 744 were built, the last of those in 1962. The Air Force maintains 58 B-52s in the active force and 18 in the Reserve. A single B-52 can carry 70,000 pounds of mixed munitions, including bombs, missiles and mines. The eight-engine jets have a range of 8,800 miles. Hide Caption 1 of 24 Photos: In the U.S. Air Force fleet C-130 Hercules transport – A C-130J Super Hercules from the 37th Airlift Squadron flies over Normandy, France, June 3, 2015. First delivered to the Air Force in 1956, the C-130 remains one of the service's most important airlift platforms. More than 140 are still in active units, with more than 180 in the National Guard and a hundred more in the Reserve. The C-130 is powered by four turboprop engines. Hide Caption 2 of 24 Photos: In the U.S. Air Force fleet AC-130 gunships – The AC-130H Spectre and the AC-130U Spooky gunships are designed for close air support, air interdiction and force protection. Armaments on the Spectre include 40mm and 105mm cannons. The Spooky adds a 25mm Gatling gun. Hide Caption 3 of 24 Photos: In the U.S. Air Force fleet F-22 Raptor – The twin-engine F-22 stealth fighter, flown by a single pilot and armed with a 20mm cannon, heat-seeking missiles, radar-guided missiles and radar-guided bombs, can perform both air-to-air and air-to-ground missions. The service has 183 of the Raptors, which went operational in 2005. Hide Caption 4 of 24 Photos: In the U.S. Air Force fleet The single-engine F-35A is the Air Force's eventual replacement for the F-16 and the A-10. The supersonic jets, which will be able to conduct air-to-air and air-to-ground attacks, are just beginning to enter the Air Force fleet. Here, an F-35 Lightning II from the 34th Fighter Squadron at Hill Air Force Base, Utah, flys at Royal Air Force Lakenheath, England, April 15, 2017. Hide Caption 5 of 24 Photos: In the U.S. Air Force fleet B-2 Spirit bomber – The four-engine B-2 heavy bomber has stealth properties that make it hard to detect on radar. Flown by a crew of two, it has an unrefueled range of 6,000 miles and can deliver both conventional and nuclear bombs. Twenty B-2s are in the active inventory. They joined the fleet in 1997. Hide Caption 6 of 24 Photos: In the U.S. Air Force fleet B-1B Lancer bomber – The four-engine jet can fly at 900 mph and carry the largest payload of bombs and missiles in the Air Force inventory. The Air Force has 62 B-1Bs in the fleet. Hide Caption 7 of 24 Photos: In the U.S. Air Force fleet F-15 Eagle – The F-15 Eagle, the Air Force's main air superiority fighter, became operational in 1975. With a crew of one or two, depending on the model, the twin-engine jets are armed with a 20mm cannon along with Sidewinder or AMRAAM missiles. The Air Force lists 249 F-15 Eagles in its inventory. Hide Caption 8 of 24 Photos: In the U.S. Air Force fleet F-15E Strike Eagle – The Strike Eagle is a version of the air superiority fighter adapted to perform ground-strike missions. With a crew of two, the twin-jet can carry and deploy most weapons in the Air Force inventory and operate in any weather. The F-15E was first delivered in 1988. The Air Force lists 219 in its fleet. Hide Caption 9 of 24 Photos: In the U.S. Air Force fleet A-10 Thunderbolt – The A-10 Thunderbolt jets, nicknamed "Warthogs," are specially designed for close air support of ground forces. Key to their armaments is a 30mm Gatling gun. The pilot is protected from ground fire by titanium armor, and the plane's fuel cells are self-sealing in case of puncture. Hide Caption 10 of 24 Photos: In the U.S. Air Force fleet RC-135U – The RC-135U Combat Sent, based at Offutt Air Force Base, Nebraska, provides strategic electronic reconnaissance information to the president, secretary of defense, Department of Defense leaders and theater commanders. Hide Caption 11 of 24 Photos: In the U.S. Air Force fleet An F-15 Eagle takes off from the Andersen Air Force Base, Guam, flight line as two E-3 Sentries are seen in the background. Hide Caption 12 of 24 Photos: In the U.S. Air Force fleet OV-10 Bronco – A 21st Tactical Air Support Squadron OV-10 Bronco aircraft fires white phosphorus rockets to mark a target for an air strike during tactical air control training. Hide Caption 13 of 24 Photos: In the U.S. Air Force fleet A-29 Super Tucano – An A-29 Super Tucano taxis on the flightline during its first arrival, Sept. 26, 2014, at Moody Air Force Base, Georgia. Afghan Air Force pilots trained on the planes that will be used in air-to-ground attack missions in Afghanistan. Hide Caption 14 of 24 Photos: In the U.S. Air Force fleet KC-135 Stratotanker – The four-engine KC-135 joined the Air Force fleet in 1956 as both a tanker and cargo jet. It can carry up to 200,000 pounds of fuel and 83,000 pounds of cargo and passengers in a deck above the refueling system. More than 400 of the KC-135s are flown by active, Air Guard and Reserve units. Hide Caption 15 of 24 Photos: In the U.S. Air Force fleet F-16 Fighting Falcon – The single-engine jet is a mainstay of the Air Force combat fleet. It can perform both air-to-air and air-to-ground missions with its 20mm cannon and ability to carry missiles and bombs on external pods. More than 1,000 F-16s are in the Air Force inventory. Hide Caption 16 of 24 Photos: In the U.S. Air Force fleet C-17 Globemaster transport – The four-engine jet joined the Air Force fleet in 1993 with a primary mission of troop and cargo transport. Each plane can carry up to 102 troops or 170,900 pounds of cargo. The Air Force has 187 C-17s on active duty, 12 in the Air National Guard and 14 in the Reserve. Hide Caption 17 of 24 Photos: In the U.S. Air Force fleet C-5 Galaxy transport – The C-5, with a wingspan of 222 feet, a length of 247 feet and a height of 65 feet, is the largest plane in the Air Force inventory and one of the largest aircraft in the world. The first versions of the four-engine jet joined the force in 1970. The Air Force expects to have 52 versions of the latest model, the C-5M, in the fleet by 2017. Hide Caption 18 of 24 Photos: In the U.S. Air Force fleet CV-22 Osprey – The Osprey is a tiltrotor aircraft that combines vertical takeoff, hover and landing qualities of a helicopter with the normal flight characteristics of a turboprop aircraft, according to the Air Force. It is used to move troops in and out of operations as well as resupply units in the field. The Air Force has 33 Ospreys in inventory. Hide Caption 19 of 24 Photos: In the U.S. Air Force fleet E-3 Sentry AWACS – AWACS stands for airborne warning and control system. This four-engine jet, based on a Boeing 707 platform, monitors and manages battle space with its huge rotating radar dome. The planes have a flight crew of four supporting 13 to 19 specialists and controllers giving direction to units around the battle space. The Air Force has 32 E-3s in inventory. Hide Caption 20 of 24 Photos: In the U.S. Air Force fleet KC-10 Extender – Based on the DC-10 passenger jet, the triple-engine KC-10 is a gas station in the sky with the ability to carry 75 people and 170,000 pounds of cargo. In its six tanks, the KC-10 can carry up to 356,000 pounds of fuel and dispense it while airborne. The Air Force has 59 KC-10s on active duty. Hide Caption 21 of 24 Photos: In the U.S. Air Force fleet T-38 Talon – The twin-engine jet trainer, used by the Air Force to prepare pilots for the F-15E Strike Eagle, F-15C Eagle, F-16 Fighting Falcon, B-1B Lancer, A-10 Thunderbolt and F-22 Raptor, first flew in 1959. Almost 550 are in the active force. Hide Caption 22 of 24 Photos: In the U.S. Air Force fleet U-2 – The single-engine, single-pilot U-2 is used for high-altitude reconnaissance and surveillance. Flying at altitudes around 70,000 feet, pilots must wear pressure suits like those worn by astronauts. The first U-2 was flown in 1955. The planes were used on missions over the Soviet Union during the Cold War, flying too high to be reached by any adversary. The Air Force has 33 U-2s in its active inventory. Hide Caption 23 of 24 Photos: In the U.S. Air Force fleet WC-135 Constant Phoenix – The four-engine WC-135 is used to fly through airspace to detect the residue of nuclear blasts. "The aircraft is equipped with external flow-through devices to collect particulates on filter paper and a compressor system for whole air samples collected in holding spheres," the Air Force says. It has two of these jets in the active force. Hide Caption 24 of 24

Newly confirmed Secretary of Defense James Mattis has already outlined readiness, the increase of personnel numbers and recapitalization as some of the administration's top priorities.

But according to Smith, the Trump administration and Congress will be faced with major challenges when it comes to fulfilling campaign promises like building a 355-ship Navy or adding more planes to the Air Force.

"You can fantasize about spending as much money as you want, but the reality is we won't be able to fix our military readiness problems without repealing the Budget Control Act," Smith told CNN. "So far the Republicans haven't been able to show us any plan to do that."