"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.

"The causes of the incident are being investigated," he added.

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.

Gas leaks are a frequent cause for explosions in Soviet-era buildings in Russia, although authorities gave no immediate indication this was the cause.