Three US diplomats visiting Russia were removed from the ship on October 14 in Severodvinsk because they were not allowed to go to the site of the nuclear submarine.

Russian officials checked the papers on three people, including navy and defense attaches, and released them. They are suspected of violating rules against foreigners visiting restricted areas, an unnamed Russian law enforcement source said.

The US says its officials have acted in accordance with the rules. "US diplomats are on an official visit and have informed the Russian authorities of their schedule," a State Department spokesman told AFP.

The Russian Foreign Ministry confirmed in a statement to the media that this was "an official visit and the US side informed the Defense Ministry in advance of the plan".

However, Moscow said that the diplomats had a different destination and were "clearly lost". "Their intention was to go to Arkhangelsk but they boarded the ship to Severodvinsk," the Russian Foreign Ministry said. These two places are about 30 km apart.

Severodvinsk is an Arctic port with a nuclear submarine factory. Some Russian cities, especially those involving nuclear and military activities, only allow foreigners to visit if they have a special permit. Severodvinsk is located near the shooting range in Nyonoksa. On August 8, 5 people were killed in a rocket engine test here.

Russia-US relations have been strained over alleged Russian meddling in the 2016 presidential election. Moscow last month criticized Washington for refusing to issue a visa to a Russian delegation to attend a meeting of the United Nations General Assembly.

The location of Nyonoksa and Severodvinsk test sites. Graphics: Fox.