MOSCOW — In online posts and calls to local officials, Russians on Monday expressed anger that the explosion of a small nuclear reactor at a military test site last week has gone unacknowledged for days by their government.

“Even if it is not as dangerous as it seems, we deserve to know,” said Danil Kotsyubinsky, a resident of St. Petersburg who has been pressing local officials for information.

The accident, which has been cloaked in secrecy, took place on Thursday at the Nenoska naval weapons range on the coast of the White Sea in northern Russia, and it apparently involved a test of a new type of cruise missile propelled by nuclear power, American analysts say.

The explosion killed at least seven people, released radiation that briefly elevated readings in a city 25 miles away and set off a scramble by Western experts to ascertain what happened. The military and a state nuclear energy company announced the deaths but few details of the accident.