Moscow — Three U.S. diplomats were taken off a train in Northern Russia on Monday and accused of traveling to a restricted area without the necessary permits. They were allegedly caught traveling from the village of Nyonoksa, near a sensitive Russian military test site where there was a deadly accident in August, to the town of Severodvinsk, which is also within the area restricted to foreigners, Russia's state-run TASS news agency reported, citing law enforcement sources.

The U.S. State Department and the U.S. Embassy in Moscow were quoted by Russian media as saying the diplomats, including U.S. military and naval attaches, were on an official business trip and had properly notified the Russian authorities about it, including the Ministry of Defense.

However, according to Russia's Foreign Ministry, the diplomats declared the city of Arkhangelsk as their destination, which is about 25 miles from Severodvinsk.

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"These (diplomats) are the embassy staff, as I understand it, the staff of the military attaché, and they enjoy (diplomatic) immunities," deputy foreign minister Yevgeny Ivanov told reporters on Wednesday. "They indeed notified us about the trip, but the destination was Arkhangelsk. Somehow they ended up near Severodvinsk."

An image provided by the Russian State Atomic Energy Corporation ROSATOM press service shows a Russian military band preparing for the funerals of Russian nuclear engineers killed in an explosion at a military test facility in a closed city some 230 miles east of Moscow, which has served as a base for Russia's nuclear weapons program since the late 1940s, Aug. 12, 2019. AP/handout

He added that the Foreign Ministry would issue a formal note of protest to the U.S. Embassy in Moscow on Thursday.

"Some administrative protocols will be drawn up, but they enjoy immunity so I think there will just be proceedings to figure out how it happened," Ivanov was quoted by TASS as saying.

The agency reported that the diplomats were being accused of committing an administrative offense for violating rules of entry and residence in Russia.

Scene of a deadly nuclear accident

In August, a nuclear accident occurred at a military testing site near the village of Nyonoksa, which is about 30 miles from Severodvinsk.

A missile engine with a nuclear power source exploded during a test, killing seven people and causing radiation levels in the area to spike.

U.S. intelligence officials said it was "likely" a failed test of the SSC-X-9 Skyfall missile, known in Russia as the 9M730 Burevestnik. President Vladimir Putin boasted in a 2018 speech about a new, experimental nuclear-powered cruise missile with virtually unlimited range and the ability to penetrate U.S. missile defense systems.

Burevestnik is one of the six new strategic weapons Russian President Vladimir Putin announced in March 2018.