U.S.-based nuclear experts said on Friday they suspected an accidental blast and radiation release in northern Russia this week occurred during the testing of a nuclear-powered cruise missile vaunted by President Vladimir Putin last year.

The Russian Ministry of Defense, quoted by state-run news outlets, said that two people died and six were injured on Thursday in an explosion of what it called a liquid propellant rocket engine. No dangerous substances were released, it said. Russia's state nuclear agency Rosatom said early on Saturday that five of its staff members died.

A spokeswoman for Severodvinsk, a city of 185,000 near the test site in the Arkhangelsk region, was quoted in a statement on the municipal website as saying that a "short-term" spike in background radiation was recorded at noon Thursday. The statement was not on the site on Friday.