"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.

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

Three people received non-life-threatening injuries and were receiving medical treatment, it added.

Initial reports said that four cadets were injured in the blast.

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.

A cadet who gave her name only as Natalya said "We are being evacuated."

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

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.

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