The United States believes last week's deadly explosion in Russia was associated with the Kremlin's hypersonic cruise missile programme, a senior Trump administration official said on Tuesday.
The US official, speaking on condition of anonymity, said Washington was not prepared to say at this point whether it was a nuclear explosion but believed it did involve radioactive elements.
The explosion could represent a potentially significant setback to the Russian programme although it remained unclear whether it was caused by a launching failure, the official told reporters.
Russian attempts to develop hypersonic cruise missiles raised questions about whether the so-called New Start nuclear agreement, which is due to expire in early 2021, should be extended for five years, the official added.
The Kremlin boasted on Tuesday it was winning the race to develop new cutting edge nuclear weapons despite a mysterious rocket accident last week in northern Russia that caused a temporary spike in radiation levels.
It has pledged to keep developing new weapons regardless, portraying the men who died in the test as heroes.
"Our president has repeatedly said that Russian engineering in this sector significantly outstrips the level that other countries have managed to reach for the moment, and it is fairly unique," Kremlin spokesman Dmitry Peskov said.
Denial of radioactive contamination
The Russian military on Tuesday told residents of a village near a navy testing range to evacuate, but cancelled the order hours later, adding to the uncertainty and confusion fuelled by last week's missile explosion that led to a brief spike in radiation that frightened residents and raised new questions about the military's weapons program.
Initially, the military told residents of Nyonoksa, a village of about 500, to move out temporarily, citing unspecified activities at the range. But a few hours later, it said the planned activities were cancelled and rescinded the request to leave, said Ksenia Yudina, a spokeswoman for the Severodvinsk regional administration.
Local media in Severodvinsk said Nyonoksa residents regularly receive similar temporary evacuation orders usually timed to tests at the range.
And just as the Severodvinsk administration reported a brief spike in radiation levels, the Defence Ministry insisted that no radiation had been released — a blunt denial reminiscent of Soviet-era attempts to cover up disasters that added to public nervousness.
Local emergency officials announced that after taking ground samples from around the area that they have found no trace of radioactive contamination.
TRT World's Simon McGregor-Wood reports.
Radiation level surge
Radiation levels in the Russian city of Severodvinsk rose by up to 16 times on August 8 after an accident that authorities said involved a rocket test on a sea platform, Russia's state weather agency said on Tuesday, the TASS news agency reported.
The Russian Defence Ministry initially said background radiation had remained normal after the incident on Thursday, but city authorities in Severodvinsk in northern Russia said there had been a brief spike in radiation levels.
Greenpeace has said radiation levels rose by 20 times.
Russia's state weather agency, Rosgidromet, said on Tuesday that it believed radiation levels had risen by four to 16 times.
President Donald Trump said Monday that the United States is learning "much" from a deadly blast during a Russian missile test that caused elevated radiation levels.
"The United States is learning much from the failed missile explosion in Russia. We have similar, though more advanced, technology," Trump wrote on Twitter.
"The Russian 'Skyfall' explosion has people worried about the air around the facility, and far beyond. Not good!" the president tweeted.
Experts have linked the blast, which killed at least five people, to the 9M730 Burevestnik nuclear-powered cruise missile, known by NATO as SSC-X-9 Skyfall and touted by President Vladimir Putin earlier this year.
Trump's assertion that the US had similar technology was quickly challenged by American expert Joe Cirincione.
"This is bizarre. We do not have a nuclear-powered cruise missile program," Cirincione tweeted.
"We tried to build one, in the 1960s, but it was too crazy, too unworkable, too cruel even for those nuclear nuts Cold War years," he wrote.
Russian Defence Ministry initially reported the explosion at the navy's testing range near the village of Nyonoksa in the northwestern Arkhangelsk region killed two people and injured six others.
The state-controlled Rosatom nuclear corporation then said over the weekend that the blast also killed five of its workers and injured three others. It's not clear what the final toll is.
Thousands of people attended the funerals on Monday of five Russian nuclear engineers killed by an explosion as they tested a new rocket engine, a tragedy that fuelled radiation fears and raised questions about a secretive weapons program.
The engineers, who died Thursday, were laid to rest Monday in Sarov, which hosts Russia's main nuclear weapons research centre, where they worked. Flags flew at half-staff in the city 370 kilometres (230 miles) east of Moscow that has been a base for Russia's nuclear weapons program since the late 1940s.
The coffins were displayed at Sarov's main square before being driven to a cemetery.