Three US diplomats were taken off a train on Monday while heading to an Arctic city near the site of a military testing accident in northern Russia.
The trio were bound for the port of Severodvinsk, where access is restricted for foreign nationals.
Severodvinsk hosts a naval shipyard and is close to a rocket test site where a deadly explosion in August led to a spike in radiation levels.
Russia said the three had stated they would visit the city of Arkhangelsk.
That city, the regional capital, is about 30km (21 miles) away.
In a statement, Russia’s foreign ministry said the US diplomats were on an official trip and had informed the Russian defence ministry of their plans.
“Only, they said their intention was to visit Arkhangelsk and they ended up en route to Severodvinsk,” it added.
“They probably got lost. We are ready to give the US embassy a map [of Russia].”
A spokesman for the US state department in Washington said: “The American diplomats were on official travel and had properly notified Russian authorities of their travel.”
Russia’s state nuclear agency Rosatom acknowledged that five of its nuclear workers were killed in the 8 August explosion during a nuclear-powered rocket engine test near the village of Nyonoksa, to the west of Severodvinsk.
Radiation levels in Severodvinsk rose 16 times above normal after the accident, Russia’s weather service reported.
The radiation would have posed little threat to humans, nuclear experts said.