Russia's Investigative Committee, which probes major crimes, said it had opened an investigation into a potential fire safety violation.

"It has been preliminarily established that there are casualties as a result of the incident," it said.

Gas leaks are a frequent cause for explosions in Soviet-era buildings in Russia, where gas is often used for heating and cooking.

Authorities gave no immediate indication that the collapse happened because of a gas leak.

"I have instructed the security forces to assist the military in clearing the rubble and rescuing victims following the collapse of a military police building on the territory of a military unit in Sertolovo," governor Alexander Drozdenko said.

Sertolovo is a small village just outside Saint Petersburg, Russia's second-biggest city.

Local media outlet 47news reported that three people died in the incident, citing emergency services.

It reported that the collapse was caused by an "explosion".

The outlet also published an unverified photo purporting to show the building, a grey three-storey block with damage visible on at least two floors.

Russia has been regularly hit with sabotage attacks on military bases and civilian infrastructure since the start of its Ukraine offensive nearly four years ago.