Russian officials say four suspected members of an illegal armed group were killed by security forces in an operation at an apartment building in St. Petersburg.

Investigative Committee spokesman Vladimir Markin said the members of Russia's Federal Security Service (FSB) went to the apartment on August 17 to apprehend people recently added to a federal wanted list.

The individuals were added to the list for alleged participation with militant groups in the volatile North Caucasus region, Markin said.

He said the suspects posed resistance to the FSB agents and opened fire but were killed in a shoot-out.

Witnesses said gunshots and explosions were heard at the scene.

Markin said no civilians or security forces were injured in the incident.

People who were evacuated from the apartment building have been allowed back into their homes.

Based on reporting by TASS, Interfax, and Meduza