Russian strategic forces carried out a large-scale surprise military drill on Wednesday, launching four nuclear missiles that were closely monitored by U.S. intelligence agencies, U.S. officials said.

The drill began around 9:00 am ET and included the test launch of two land-based intercontinental ballistic missiles (ICBMs) and two submarine-launched ballistic missiles (SLBMs).

The test firings were unusual because of the number of missiles fired at one time, said officials who discussed some details of the drill on condition of anonymity.

State Department spokeswoman Alexandra F. Bell confirmed the tests and said the long-range missile firings were "conducted consistent with the requirements of the New START Treaty."

At the Pentagon, spokeswoman Cynthia O. Smith said: "With regard to Russia's recent testing of its strategic forces missiles, the United States received the proper notifications prior to the launches."

Other officials said the tests were notified in advance to the U.S. government as required by a 1988 U.S.-Soviet agreement requiring advance notification of ICBM and SLBM launches. The agreement requires notifying the Nuclear Risk Reduction Center, run by the State Department, of the impending launches within four days of their launch.

Mark Schneider, an arms control specialist, said Russian reports of the exercises described the test firings as a "a major strategic nuclear exercise involving a nuclear war." The exercises also involved Russian President Vladimir Putin as a commander.

Schneider, a policy analyst at the National Institute for Public Policy, said the last similar Russian strategic exercise took place two weeks before the 2012 presidential election and were described as the first time Putin personally tested an automated communication and management system for nuclear forces.

"Minimum deterrence advocates like to ignore these exercises," Schneider said. "I think they are very important and point out the need to maintain a strong triad" of missiles, submarines, and bombers.

The missiles test fired were identified by the officials with their NATO code names, including a silo-based SS-18 and a road-mobile SS-25, along with the two submarine-launched missiles, an SS-N-18 and SS-N-23.

The SS-18 is Russia’s largest land-based ICBM with a range of up to 10,000 miles and up to 10 warheads, or multiple, independently-targetable reentry vehicles (MIRVs).

The SS-25 is a road-mobile missile capable of launching up to four MIRVs and has a range of up to 6,200 miles.

The SS-N-18 Stingray missile can be equipped with up to seven warheads and is carried on Delta III missile submarines. It has a range of up to 5,000 miles.

The SS-N-23 can carry up to 10 MIRVs and has a range of up to 5,000 miles. It is deployed on Delta IV missile submarines.

A Kremlin spokesman in Moscow said the "surprise check" of nuclear forces, aerospace defenses, and naval and strategic aviation were ordered by "Commander-in-Chief, President Vladimir Putin."

"Ballistic missiles were launched, the air defense and missile defense systems were used," Dmitri Peskov, the spokesman, was quoted by Russian news agencies as saying. "All launches took place as expected. All of the practice targets were hit."

Other reports identified the exercise as a "snap inspection."

The SS-25, which Russia calls the Topol, was launched from Plesetsk and the SS-18, called RS-20V, was launched from Dombarovski located near the border with Kazakhstan in Central Asia.

Additionally, Russian forces fired four short-range SS-26 and SS-21 short-range missiles from the Kapustin Yar test center near the Black Sea. Those missiles traveled over 60 miles.

Air defense forces also fired S-400 and S-300 anti-aircraft and anti-missile interceptors at incoming ballistic missile targets at the Kapustin Yar test range.

The strategic missile exercises highlight Moscow’s large-scale nuclear forces build up under Putin.

Russia is developing several new missiles, including a weapon U.S. intelligence agencies have assessed as a covert intermediate-range nuclear missile called the RS-26 that is being developed and tested in apparent violation of the 1987 Intermediate-range Nuclear Forces Treaty.

The Russian government has denied the RS-26 violates the INF treaty and claims it is a new ICBM, which some arms compliance experts say is a violation of the 2010 New START treaty.

Russia is also developing a new submarine-launched ballistic missile and a new class of missile submarines.