Washington (CNN) Russian military aircraft were spotted flying off the coast of Alaska for the fourth time in as many days, a spokesperson for the North American Aerospace Defense Command told CNN on Friday.

The two most recent sightings occurred late Wednesday and on Thursday, with the first involving two IL-38 maritime patrol aircraft and the second involving two Tu-95 nuclear-capable Bear bombers.

Russian aircraft never entered US airspace but US F-22s and Canadian CF-18s jets were dispatched to perform an intercept during Thursday's encounter, according to NORAD.

"Obviously -- we are aware of it," White House press secretary Sean Spicer told reporters on Friday. "This is not highly unusual ... but we monitor everything."

On Thursday, the bombers entered the Alaskan Air Defense Identification Zone 700 nautical miles southwest of Anchorage -- significantly farther from the US coastline than two other encounters that occurred on Monday and Tuesday.

The Alaskan Air Defense Identification Zone is a designated region of international airspace, primarily surrounding the US and Canada, that is meant as a buffer to allow for the identification of aircraft heading towards North America.

While these flights pose no real military threat, US defense officials are taking notice of the high frequency at which they've occurred this week.

There is "no other way to interpret this other than as strategic messaging," one official told CNN.

While the Russians have not conducted flights of this nature since 2015, another senior defense official stressed that they are "not a concern" and attributed the uptick to a recent lack of available Russian aircraft and need to boost training.

"We haven't seen this sort of level of activity for a couple of years," said John Cornelio, a NORAD spokesperson, though he emphasized it was not "unprecedented" or "unusual."

This "shows the value of NORAD and that binational US and Canada relationship," he said, pointing to the two nations working together to identify and intercept the Russian long-range aircraft.

Earlier in the week, US defense officials called recent sightings of the bombers "nothing out of the ordinary" -- itself an indication that both nations are toeing the line between routine military posturing and escalating provocation.

On Monday , US F-22 fighter jets intercepted two Russian bombers in international airspace 100 miles from Kodiak Island, Alaska. A US military official called the interaction "safe and professional."

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

Less than 24 hours later, a US surveillance aircraft responded to two Russian bombers that were spotted in the same area, this time flying 41 miles off Alaska.

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The US itself has carried out similar flights along both the Chinese and Russian coasts.

Part of larger strategy

Moscow, for its part, said it "regularly carries out patrol missions above the neutral waters of the Arctic, the Atlantic, the Black Sea and the Pacific Ocean."

"All such missions are carried out in strict compliance with international regulations and with respect to national borders," the Russian Defense Ministry said in a written statement.

But this week's encounter plays into a larger effort by Russian President Vladimir Putin "to prove Russia is back in the game," according to Howard Stoffer, a former State Department staffer.

"This kind of cat-and-mouse stuff has been going on for a while now," Stoffer told CNN, adding that Putin "is trying to put the US on notice that the Russians are everywhere and are back to expanding the limits of expanding their military power."

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"It is one thing when you fly to be noticed," he said. "When the Russians buzz US ships, that is an unprofessional action because upsets the operation and is dangerous for all parties involved ... that is where the line that is drawn."

US officials have echoed Stoffer's stance as recently as February, after the USS Porter had three encounters with Russian aircraft while sailing in the Black Sea.

Those encounters were deemed unsafe and unprofessional because of how close the Russian planes flew to the American destroyer, a senior defense official said at the time.

Moscow denied that its aircraft had made any unsafe moves.

Russian aircraft have also been spotted recently flying near the coastline of US allies, including Japan, which has scrambled fighter jets four times this month in response, according to a statement from the Japanese Foreign Ministry.

Photos: Photos: F/A-18 Hornets fly above the Nimitz-class aircraft carrier USS John C. Stennis in the Pacific Ocean. The US Navy has 10 of the 97,000-ton ships, which can carry more than 60 aircraft each. Hide Caption 1 of 10 Photos: An F-35B Joint Strike Fighter tests its vertical landing capability in 2015. The fifth-generation jets have been deployed to Marine Corps Air Station Iwakuni in Japan as the US moves some of its most modern equipment to Northeast Asia. Hide Caption 2 of 10 Photos: Chinese J-20 stealth fighters perform during the 11th China International Aviation and Aerospace Exhibition, also known as Airshow China 2016, in Zhuhai on November 1, 2016. The J-20 is one of China's answers to US F-22 and F-35 stealth fighters. Hide Caption 3 of 10 Photos: Chinese military vehicles carrying DF-26 ballistic missiles participate in a military parade at Tiananmen Square in Beijing in 2015. The missile has been dubbed "the Guam killer" by analysts for their ability to hit the strategic US island in the Pacific. Hide Caption 4 of 10 Photos: Korean People's Army (KPA) soldiers march on Kim Il Sung sqaure during a military parade in Pyongyang on April 15, 2017. North Korea trails only China, the US and India in the number of people in the military. Hide Caption 5 of 10 Photos: A North Korean tank participates in a competition in this 2017 picture released from North Korea's official Korean Central News Agency. North Korea has 3,500 main battle tanks in its arsenal. Hide Caption 6 of 10 Photos: The South Korean Navy's first Aegis destroyer "King Sejong" is seen at the launching ceremony at the Ulsan dockyard of Hyundai Heavy Industries in the southeastern port city of Ulsan in 2007. The 7,600-ton destroyer is considered by analysts to be among the world's most advanced warships. Hide Caption 7 of 10 Photos: A South Korean K2 tank (C) moves during an equipment demonstration at the Defense Expo Korea 2016 at KINTEX exhibition hall in Goyang, north of Seoul, on September 10, 2016. The K2, nicknamed the "Black Panther" is considered to be among the world's best tanks. Hide Caption 8 of 10 Photos: The Japanese submarine Oyashio, escorted by a Japanese destroyer, arrives at the former US naval base in Subic Bay, Philippines, in April 2016. Submarines are considered a key strength of one of the world's strongest navies. Hide Caption 9 of 10 Photos: Japan's Maritime Self-Defense Force helicopter destroyer Izumo takes part in a fleet review off Sagami Bay in October 2015. Helicopter destroyers combine with Japanese subs to give Tokyo exceptional anti-submarine warfare capabilities. Hide Caption 10 of 10

The Viktor Leonov, a Russian spy ship , has also been spotted near the US coastline twice in recent months.

Rising tensions between US and Russia are a far cry from President Donald Trump's optimistic campaign rhetoric of hopes for a collaborative relationship. As Trump himself said earlier this month, relations between the former Cold War foes "may be at an all-time low."

Syrian President Bashar al-Assad's alleged April 4 chemical weapons attack on his own civilians triggered Trump's outrage, leading him to strike a Syrian airfield with Tomahawk missiles, and witnessed a change in Trump's stance on Russia , which has supported Assad throughout Syria's bloody civil war.

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Russian Foreign Minister Sergey Lavrov said that the US missile strikes on an airfield in Syria were a failed attempt to try to undermine the peace process in the country and to change the regime.

And Russia has stood with Iran, a long-time US foe, in condemning the strikes.

Photos: Photos: Russian Foreign Minister Sergey Lavrov speaks on Friday, April 7, during a news conference in Tashkent, Uzbekistan. Responding to a US missile strike on a Syrian airbase , he said, "I am particularly disappointed by the way this damages US-Russia relations. I don't think this will lead to an irreversible situation." Hide Caption 1 of 10 Photos: German Chancellor Angela Merkel attends a refugee relief panel at an event honoring volunteers in Berlin. In a statement Friday Merkel said, "This attack by the United States of America is understandable, given the aspect of the war crimes, given the suffering of innocent people and given the logjam in the UN Security Council." Hide Caption 2 of 10 Photos: Japanese Prime Minister Shinzo Abe speaks to reporters in Tokyo. "The Japanese government supports the US government's resolve that it will never tolerate the spread and use of chemical weapons," he said. Hide Caption 3 of 10 Photos: Copies of the Japanese daily newspaper Nikkan Gendai at a railway station in Tokyo show pictures of President Trump. Hide Caption 4 of 10 Photos: Turkish Foreign Minister Mevlut Cavusoglu speaks to the media in Antalya, Turkey. He welcomed Friday's US airstrike on Syria, according to a statement. Hide Caption 5 of 10 Photos: Australian Prime Minister Malcolm Turnbull speaks during a news conference in Sydney. He said Australia "strongly supports the swift and just response of the US" to the recent chemical attack in Syria's Idlib province. He added that Australia was "not involved in the strike" but was informed by the US about the action shortly before it was carried out. Hide Caption 6 of 10 Photos: Italian Prime Minister Paolo Gentiloni speaks at a news conference in Rome. The country's foreign minister, Angelino Alfano, said in a statement that the US military action in Syria was "proportionate and well-timed." Hide Caption 7 of 10 Photos: French President Francois Hollande delivers a speech in Annonay, France, about the situation in Syria. United Nations action is required to address the conflict in Syria and to prevent the use of chemical weapons, he said, adding that he hopes negotiations might still lead to a peaceful transition. Hide Caption 8 of 10 Photos: Iranians shout anti-US slogans after a Friday prayer ceremony in Tehran, following a US airstrike in Syria. Iranian Foreign Ministry spokesman Bahram Qasemi said Iran "strongly condemns" President Trump's military strike against the Syrian government, according to Iran's semi-official news agency ISNA. Iran also "condemns any use of chemical weapons no matter who uses it or who the victims are," he said. Hide Caption 9 of 10 Photos: Czech Prime Minister Bohuslav Sobotka holds a news conference in Prague, Czech Republic. Sobotka expressed his support for President Trump's launch of missile strikes. Hide Caption 10 of 10

"Attempts of this kind will never be a success. It will never happen," Lavrov said during a joint news conference with the Iranian and Syrian foreign ministers in Moscow. "We demand that the United States should respect the sovereignty of a state and avoid such actions that threaten the current world order."

Opposing views on the conflict in Ukraine have also become a hot-button issue between the Kremlin and the new US administration.

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In February, Russia's Foreign Ministry also indicated that it intends to keep Crimea and not return it to Ukraine because it considers it to be part of Russia -- a stance that the Trump administration has said it directly opposes.

White House press secretary Sean Spicer said earlier this year that Trump had been taking a tough line with Russia and that he expected Moscow to withdraw from the region, which it has occupied since 2014

The US, meanwhile, has positioned military assets across Europe in an effort to reassure its European and NATO allies in the wake of Russia's movements in Ukraine.

Over the weekend, the US Air Force sent its newest stealth fighters to the United Kingdom in a demonstration of its military reach.

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This week's encounters might be routine military chest-thumping, but the countries' entwinement in complex military situations around the world raises the risk of escalation.

American forces have so far refrained from engaging Russian aircraft after they've performed maneuvers like buzzing Navy ships. But Stoffer indicated retaliation could be possible in the future.

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According to Stoffer, it is unlikely that the US would go to the extreme of firing a shot across the bow of a Russian ship or shooting down a Russian jet carrying out an unsafe move.

But he could see a scenario in which a US commander greenlights alternative responses like jamming the aircraft's radar and avionics systems -- which could cause the aircraft to crash.

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If a minor provocation were to escalate and turn into a larger-scale war situation, Moscow would be at a disadvantage, according to retired Air Force Gen. Michael Hayden.

"No one wants to go to war with the Russians, but let me double down on another concept: The Russians really don't want to go to war with us," said Hayden, the CIA and National Security Agency director under President George W. Bush, during an interview on CNN's "Erin Burnett OutFront ."

"They are by far the weaker power," he said.