Russia has admitted that an explosion that killed five scientists last week happened during testing of a small nuclear power reactor – as western experts have suspected.

The country’s leading nuclear official promised on Monday to succeed in developing new weapons despite the accident in what was thought to have been a failed test of a new missile.

Russia’s defence ministry had initially said the explosion, on an offshore platform in the Arkhangelsk region near the Arctic Circle, had happened during testing of a liquid-fuelled rocket engine, and had killed two people.

But nuclear agency Rosatom later said several scientists had been killed during testing of an “isotope power source in a liquid propulsion system”.

US nuclear experts believe Rosatom may have been testing an experimental nuclear-powered cruise missile, which president Vladimir Putin last year claimed to be “invincible against all existing and prospective” defence systems.

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 men “tragically died while testing a new special device,” Alexei Likhachev, Rosatom chief executive, said at their funeral in Sarov, a high-security city devoted to atomic research less than 250 miles east of Moscow.

An official at the Russian Federal Nuclear Centre in Sarov, where the men worked, suggested they had been working on a small nuclear reactor, work “many countries” were doing.

Valentin Kostyukov, head of the centre, which is part of Rosatom, said the test had been preceded by a year of careful work and a state commission was investigating what went wrong.

The nuclear experts battled to control the situation, but were unable to prevent the accident, Mr Kostyukov said.

Vyacheslav Soloviev, scientific director of the institute, said that their part of it was developing small-scale power sources that use “radioactive materials, including fissile and radioisotope materials”.

Although the defence ministry initially said no change in radiation was detected after Thursday’s explosion, officials in the nearby city of Severodvinsk said radiation had briefly spiked, which prompted US nuclear experts to suspect the failed test involved a nuclear-powered cruise missile.

The experts suspected it was the Burevestnik missile, one of an array of new strategic weapons touted by Mr Putin last year.

Anxious local residents stocked up on iodine, used to reduce the effects of radiation exposure.

Tensions between Moscow and Washington over arms control have been exacerbated by the demise this month of a landmark nuclear treaty.

A series of deadly accidents have damaged the Russian military’s reputation, including huge explosions earlier last week at a Siberian depot that killed one person and injured 13.

Last month 14 sailors died in a fire aboard a nuclear-powered submarine in the Barents Sea. A top naval official later said the men gave their lives preventing a “planetary catastrophe”.

Additional reporting by agencies