Russia's Foreign Ministry says it will issue a formal note of protest to the U.S. Embassy in Moscow after Russian authorities caught three U.S. diplomats in a restricted area near a secret test site in northern Russia.

Axar.az reports citing TASS that the trio, which included the U.S. military and naval attachés, was removed from a train on October 14 and briefly questioned by Russian authorities in the sensitive Arctic shipyard city of Severodvinsk, near the site of a mysterious explosion in August that killed five nuclear workers.

A U.S. State Department spokesman said the diplomats had been on an official trip and that they had notified Russian authorities in advance of their travel plans. The reason for the diplomats' travel was not disclosed.

But Russia's Foreign Ministry said the diplomats had been found in a restricted area more than 40 kilometers from Arkhangelsk, the city they had said they planned to visit.