Russia removed three American diplomats from a train heading to an Arctic town near the site of a recent nuclear accident, Russian news agencies reported on Wednesday.

The removal occurred on Monday in the shipyard town of Severodvinsk, according to Interfax.

The diplomats were about to take a train to Nyonoksa, the village closest to the military testing site where a mysterious explosion in August left seven people dead and caused a spike in radiation levels miles away.

Both Nyonoksa and Severodvinsk, which stand about 25 miles apart, are closed cities that require foreigners to obtain a special permit from the authorities before arriving. Russians must also gain special approval to enter Nyonoksa.

A State Department spokesperson said in an email on Wednesday that the American diplomats “were on official travel and had properly notified Russian authorities of their travel”.

Huge explosion at Russia ammunition depot: In pictures Show all 12 1 /12 Huge explosion at Russia ammunition depot: In pictures Huge explosion at Russia ammunition depot: In pictures An explosion at an ammunition depot near the town of Achinsk in Russia on August 5 AFP/Getty Huge explosion at Russia ammunition depot: In pictures Fire and smoke rise from the site of an explosion at an ammunition depot in Russia on August 5 Reuters Huge explosion at Russia ammunition depot: In pictures People watch from afar as as fire and smoke rise from the site of an explosion at an ammuntion depot in Russia on August 5 Reuters Huge explosion at Russia ammunition depot: In pictures Flaming residue is scattered in an explosion at at an ammunition depot near the town of Achinsk in Russia on August 5 Reuters Huge explosion at Russia ammunition depot: In pictures An explosion at an ammunitions factory is seen from 10 miles away in the town of Achinsk, Russia on August 5 AP Huge explosion at Russia ammunition depot: In pictures Fire and smoke rise from the site of an explosion at an ammunition depot in Russia on August 5 Reuters Huge explosion at Russia ammunition depot: In pictures Smoke rises from the site of an explosion at an ammunition depot as the sun sets in the town of Achinsk, Russia on August 5 AFP/Getty Huge explosion at Russia ammunition depot: In pictures An explosion at an ammunitions factory is seen from 10 miles away in the town of Achinsk, Russia on August 5 AP Huge explosion at Russia ammunition depot: In pictures Smoke rises from the site of an explosion at an ammunition depot in Russia on August 5 AFP/Getty Huge explosion at Russia ammunition depot: In pictures Smoke rises from the site of an explosion at an ammunition depot in Russia on August 5 AFP/Getty Huge explosion at Russia ammunition depot: In pictures Smoke rises from the site of an explosion at an ammunition depot in Russia on August 5 AFP/Getty Huge explosion at Russia ammunition depot: In pictures Smoke rises from the site of an explosion at an ammunition depot in Russia on August 5 AFP/Getty

The Russian foreign ministry acknowledged the fact that the diplomats had notified the Russian defence ministry about their plans.

“But they only stated their plans to visit Arkhangelsk,” the ministry said, referring to the regional capital, about 26 miles east of Severodvinsk. “Then they appeared on the Nenoksa-Severodvisnk train,” the statement continued. “Perhaps they got lost.”

The ministry also said it was ready “to present a map of Russia to the American Embassy”, Interfax said.

The Russian authorities have said very little about the accident on 8 August, which took place at a Russian navy rocket testing range on the White Sea.

Five Russian nuclear scientists and two military officers died as a result of the explosion, and the accident tripped radiation metrs in two cities, including Arkhangelsk.

Two days after the explosion, Rosatom, the Russian nuclear agency, said it occurred as a result of a test involving “isotopic sources of fuel on a liquid propulsion unit”.

The US president Donald Trump later suggested the blast involved a type of novel, nuclear-propelled cruise missile known in the West as the “Skyfall”.

Shortly after the accident, the local authorities in Severodvinsk released a statement reporting a spike in radiation levels, and the authorities announced an evacuation of Nyonoksa.

The statement was quickly removed from the city’s website and days of rumours, conflicting reports and near-total silence from the authorities followed.

The government’s response raised fears that it intended to hide the real consequences of the accident, and brought back memories of the Chernobyl nuclear disaster of 1986, when the Soviet authorities kept people in the dark about the accident for days.