A mysterious Russian military explosion that left five Russian scientists dead last week happened during tests on a new nuclear-powered rocket, it has emerged.

Experts were working on 'a nuclear isotope power source' for the weapon when the blast occurred at the Nyonoksa facility in north western Russia, officials say.

Moscow has since admitted there were two - not one as previously disclosed - spikes in background radiation near the city of Severodvinsk as a result of the accident.

It comes as thousands of people attended the burials of five Russian nuclear engineers killed by the explosion. The engineers, who died on Thursday, were laid to rest on Monday in the city of Sarov that hosts Russia's main nuclear weapons research centre.

Today, Russia's nuclear agency chief confirmed that the scientists were developing 'new weapons' and vowed to continue testing 'until the end', despite the explosion.

A mysterious Russian military explosion that left five Russian scientists dead last week happened during tests on a new nuclear-powered rocket, it has emerged. Officials were seen wearing protective clothing as they transported casualties last week (pictured)

Vyacheslav Lipshev, 40, was one of the experts killed in the blast. His widow Natalia Alexeeva, 40, posted a tribute: 'I love you my dear, how will I live without you? You are my everything.'

Software and hardware specialist Alexey Vyushin (left), 43, and Vyasheslav Yanovsky (right), 71, were both killed

One of the dead was Evgeny Korotaev, 50, a leading electronics engineer and also a popular DJ, whose second wife had given birth to twin girls just seven months ago.

'The best (thing) for their memory will be our further work on the new weapons,' the head of Rosatom Alexei Likhachev was quoted as saying by agencies.

'We are fulfilling the task of the motherland, its security will be reliably ensured,' he added.

The Defense Ministry initially said the explosion at the navy's testing range killed two people and injured a further six, but the state-controlled Rosatom nuclear concern acknowledged later that the blast also killed five of its workers and injured three others.

Pictures emerged today of the five 'elite' scientists - dubbed 'national heroes'.

Two of the men were blown into the sea at the top secret weapons testing zone in the White Sea.

Their bodies were initially lost but later found and funerals for all those killed were due to be held in top secret closed nuclear research town Sarov from where foreigners are banned.

According to one version, the troubling missile accident came as the scientists were working on the nuclear engine of deadly Burevestnik cruise missile with 'unlimited range' nicknamed the 'Flying Chernobyl' when it exploded.

One of the dead was Evgeny Korotaev, 50, a leading electronics engineer and also a popular DJ, whose second wife had given birth to twin girls just seven months ago.

Like the other dead, he worked for the classified Institute of Experimental Physics based in Sarov, 235 miles east of Moscow, known as Arzamas-16 in Soviet times.

His daughter from the first marriage, Oksana, 26, posted a childhood picture of her with her father and the caption: 'Daddy, I love you so much.'

Experts were working on 'a nuclear isotope power source' for the weapon when the blast occurred at the Nyonoksa facility in north western Russia, officials say.

Sergey Pichugin, 45, a testing engineer, was also killed. He is pictured with wife ElenaKoro

Korotaev's daughter from the first marriage, Oksana, 26, posted a childhood picture of her with her father and the caption: 'Daddy, I love you so much.'

Oksana Korataeva (pictured) is the eldest daughter of Evgney Korataev, who died in the blast

She only recently gave birth to his grandchild.

Another killed was Vyasheslav Yanovsky, 71, one of Russia's most senior nuclear scientists, deputy head of research and testing at the institute.

He was an 'honoured worker' of Moscow's nuclear industry, and died alongside Vyacheslav Lipshev, 40, head of the institute's research and development team.

Lipishev's widow Natalia Alexeeva, 40, posted a tribute: 'I love you my dear, how will I live without you? You are my everything.'

Software and hardware specialist Alexey Vyushin, 43, who had developed a high-energy photon spectrometer, and Sergey Pichugin, 45, a testing engineer, were also killed.

All are expected to be honoured posthumously by Vladimir Putin.

Valentin Kostyukov, director of the Russian Federal Nuclear Centre, belatedly acknowledged the disaster, but called the lost men 'national heroes'.

Previously Russia had admitted only one.

Nuclear centre deputy head Vyacheslav Solovyev admitted the scientists were killed by an explosion in a small nuclear reactor, part of the engine of the missile

Alexander Chernyshov, deputy scientific director of the Russian Federal Nuclear Centre, admitted there had been two surges of radiation in the aftermath of the accident

'The tragedy happened, the explosion took the lives of five our friends, and three more are hospitalised,' he said.

The lives of the three were 'not in danger', he said.

'The death of our employees is the bitter loss for our institute and for the state corporation Rosatom.

'The testers involved in this are of course the national heroes and we always believed that they are the elite of the federal nuclear centre.'

He said that preparations for the top-secret task in the White Sea had been ongoing for one year.

'But sadly the situation developed in a non-standard way and now a state commission is working to investigate the reasons,' he said.

Alexander Chernyshov, deputy scientific director of the Russian Federal Nuclear Centre, admitted there had been two surges of radiation in the aftermath of the accident.

The Nyonoska testing site, pictured above, is reportedly the location of trials for Putin's new Zircon missile

'Our people established by direct measurement that there really was - twice - an increase in background radiation,' he said.

'It lasted no longer than an hour. No residual radioactive pollution was found by our experts.'

Nuclear centre deputy head Vyacheslav Solovyev admitted the scientists were killed by an explosion in a small nuclear reactor, part of the engine of the missile.

This 'energy source' contained radioactive fissile materials, and he insisted the US was engaged in similar military developments.

'We are now trying to figure out (what went wrong), working closely with the state commission, analysing the entire chain of events in order to assess the scale of the accident,' he said.

The aim was to 'understand its causes to exclude the possibility of such accidents in future.'