The US State Department confirmed the visit -- which according to the reports took place on Monday -- but said the diplomats were on an official trip and had informed the Russian authorities.

Moscow in turn said the diplomats had named a different city as their destination and had "obviously got lost".

In a statement to Russian news agencies, the Russian foreign ministry confirmed "that this was an official trip and that they 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," the ministry said, referring to the regional capital around 30 kilometres (21 miles) away.

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

"They obviously got lost. We are ready to give the US embassy a map of Russia," the ministry added sarcastically.

According to the news agency reports, 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.

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 news agency reported.

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.

- Russians 'notified' -

But the US said the diplomats had acted in accordance with the rules.

"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.

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.