The US State Department confirmed the visit but said the diplomats were on an official trip and had informed the Russian authorities.

The diplomats were on Monday taken off a train running to Severodvinsk from Nyonoksa, the site of a missile test blast that saw a radiation spike last August, reported Interfax news agency.

The trio, including a naval attache and a defence attache, were removed from the train in Severodvinsk station at around 6:00 pm (1500 GMT) after document checks and then released, Interfax said.

TASS news agency, citing a source from the law enforcement authorities, said the diplomats were suspected of breaching rules on foreigners visiting controlled zones.

Severodvinsk does not allow foreigners to visit freely, TASS noted.

"The American diplomats were on official travel and had properly notified Russian authorities of their travel," a State Department spokesperson told AFP.

Some Russian cities, particularly those involved in military and nuclear activities, only allow foreigners to visit with special permits.

Severodvinsk is an Arctic port with a naval shipyard that builds nuclear submarines.

Nearby Nyonoksa is a missile testing site on the coast of the White Sea. On August 8, an explosion there killed five people working for Russia's nuclear agency.

The blast briefly caused radiation levels in Severodvinsk to rise to more than 16 times background levels, Russian officials said, while denying any danger to public health.

Relations between Moscow and Washington have been battered by accusations of Russian interference in the 2016 US presidential elections.

Although President Vladimir Putin and US President Donald Trump have sought to improve bilateral ties, they remain strained.

Russia last month hit out at Washington for denying visas to a Russian delegation due to attend the UN General Assembly.