Two people have been killed and radiation levels reportedly spiked after a rocket engine exploded during a test in northern Russia, prompting authorities to close a portion of the White Sea to civilian ships for a month.

The reports of a sudden rise in radiation levels in the nearby city of Severodvinsk after the explosion contradicted official statements by Russia’s defence ministry that all radiation levels in the area remained stable. The surge in radiation levels lasted less than one hour, local officials said.

The explosion took place at a military testing range in the Arkhangelsk region, where specialists were working on a liquid-fuel jet engine, the defence ministry said. Those injured included military specialists and civilian engineers for the rocket engine’s developer.

Some reports from state-run media suggested the blaze occurred onboard a vessel, while the military’s account implied it happened on land. Russia maintains a veil of secrecy around its military installations in the region, near where its northern fleet, including nuclear submarines, are stationed. Severodvinsk houses the country’s main shipyard for building and repairing nuclear submarines.

The deputy head of the Arkhangelsk port told the Interfax news agency that Russia had closed the area near the explosion to civilian ships for a month. The closure could indicate that there are contamination concerns or that Russia’s military may be conducting missions in the area as a result of the incident. A defence ministry spokesman did not immediately return several phone calls seeking comment.

Associated Press quoted emergency officials as saying the city’s residents never were in any danger despite warning all workers to stay indoors.

Early reports suggested there were heightened radiation levels near the town of Nyonoksa, where there is a site for testing ballistic missiles carried by Russia’s nuclear-powered submarines.

The defence ministry denied reports of a rise in radiation levels, saying levels were normal and there had been “no harmful discharges into the atmosphere”.

A source told the Vedomosti newspaper that the accident occurred during testing of an engine used by a sea-launched missile currently in use by the Russian navy. That would not explain the radiation surge, although liquid fuel for rockets can also be highly toxic. It would explain some heightened security measures.

This week a fire at a military ammunition depot in Siberia forced more than 16,500 people to evacuate their homes, as exploding shells sent deadly shrapnel flying for miles around. The fire left one person dead and 13 more injured, and took firefighters more than 16 hours to extinguish.

Investigators have signalled that they believe negligence and a “human factor” was to blame for the Siberian fire, opening a criminal case for the mishandling of explosives.