Russia closed off an area of the White Sea to shipping for a month after the fatal explosion of a rocket engine on a military testing ground in Russia‘s north earlier on Thursday, Interfax cited the port of Arkhangelsk as saying.

The explosion of a liquid-propellant rocket engine killed two people on Thursday in Russia‘s northern Arkhangelsk region, but released no dangerous substances into the atmosphere, the state-run RIA news agency had earlier cited the Defense Ministry as saying.

A port official said the Dvina Bay area of the White Sea would be closed to shipping.

The Defense Ministry was quoted as saying that radiation was at normal levels. It said six people were injured.

“A short-term rise in background radiation was recorded at 12 o’clock in Severodvinsk,” Ksenia Yudina, a spokeswoman for the city authorities, was quoted as saying.

City officials said background radiation levels had fully “normalized.” The population of Severodvinsk was 185,000 at the beginning of 2017, according to officials.

The rocket engine explosion occurred at a weapons testing area near the village of Nyonoksa in Arkhangelsk region, the Interfax news agency cited unnamed security sources as saying.

Russian media said an area near Nyonoksa is used for tests on weapons including ballistic and cruise missiles that are used by the Russian Navy.

“No harmful substances were emitted into the atmosphere, the background radiation is normal,” the Defense Ministry was quoted as saying.