MOSCOW (AP) — Russia’s Defense Ministry said a rocket engine exploded during a test Thursday, killing two people and injuring four others. Local officials reported a slight increase in radiation levels in a nearby city.

The ministry said the explosion occurred at a military shooting range in Nyonoksa in the northwestern Arkhangelsk region, causing a fire. It said six servicemen and civilian engineers were injured, and two of them later died of their injuries.

Some Russian news reports cited unidentified sources who said up to 15 people were injured.

Nyonoksa hosts a navy facility that serves as a base for testing intercontinental ballistic missiles intended for nuclear submarines.

The ministry said there was no release of radioactivity or any toxic substances, but the local administration in Severodvinsk, 30 kilometers (19 miles) east of Nyonoksa, reported a small, brief increase in radiation levels.

The city administration said the radiation level rose to 2 microsieverts per hour for about 30 minutes before returning to the natural level of 0.1 microsieverts per hour. In line with standard procedures, emergency officials issued a warning to all workers to stay indoors but authorities insisted that the city’s residents never were in any danger.

The radiation level of 2 microsieverts per hour is only slightly higher than the natural background radiation, which could vary between 0.1 and 0.4 microsieverts per hour. It’s lower than the cosmic radiation that plane passengers are exposed to on a trans-ocean flight.

Still, the radiation warning spooked residents, provoking intense discussions on social networks.

The explosion Thursday in Nyonoksa followed a massive fire that erupted Monday at a military ammunition depot near Achinsk in eastern Siberia’s Krasnoyarsk region. The blaze triggered powerful explosions that continued for about 16 hours, killing one person, injuring 13 others and forcing over 16,500 people to flee their homes.