"According to preliminary information, four servicemen received various injuries," it said in a statement, adding that the injuries are not life-threatening.

Earlier, the Russian defence ministry said three people were wounded in the blast.

"An explosion of an unidentified explosive device took place around 1:30 pm (1030 GMT), in Saint Petersburg in staff premises of an administrative building at the Mozhaisky academy," the ministry said in a statement cited by agencies.

It described the device as lacking casing, meaning that it was not a military device such as a mortar shell.

"Everyone is alive," Saint Petersburg's acting governor Alexander Belov told the media.

"Three people are injured, one is lightly injured," Belov said, without clarifying whether that made the total number of wounded three or four.

The Investigative Committee opened a criminal case into the incident, saying it was working to establish "the origin of the detonated object".

Police had sealed off the area around the academy, an AFP journalist saw, and cadets and staff were leaving the grounds. Fire engines were also driving out of the gates.

The TASS news agency reported that military prosecutors were inspecting the site.

A cadet who gave her name only as Natalya told AFP that "everyone is being evacuated."

She added that "it looked like something exploded on the first floor" but said she did not have any more information.

An emergency services official told RIA Novosti that the blast reverberated in a first-floor classroom, causing a staircase to collapse and trapping around 20 people.

Local independent news site Fontanka reported that mine clearing specialists were also at the scene.

There was no visible damage to the building.

The academy, one of Russia's largest, is overseen by the defence ministry and trains officers to serve in air and space defence and other branches of the armed forces.

It is located in the centre of Saint Petersburg, the former imperial capital of Russia whose palaces are a major draw for tourists.

In April 2017, a bomb went off on a train in the city's metro, killing 15 people and injuring almost 70 more. The alleged perpetrator also died in the attack.

The blast on Tuesday came as 11 went on trial in the city at a military court charged with helping to organise the 2017 suicide bombing that was claimed by a little-known group linked to al-Qaeda.