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At least five people have been hospitalised in the Russian city of St Petersburg after two explosions at two separate universities.

There was a large explosion on the second floor of the Military Space Academy at 1.30pm local time.

According to early reports four people were injured in the blast, all of whom were military officers.

A warning was sounded to evacuate before the device went off which caused a staircase to collapse.

According to reports two teachers at the academy, a man and woman, found an explosive device and decided to relocate it to a safer place before it went off.

Ten students were trapped in a room by the rubble of the collapsed staircase.

Officials from Russia's FSB spy agency were reportedly at the scene along with police and emergency rescuers.

What is officially named the A.F. Mozhaysky's Military-Space Academy is one of Russia's largest universities and was established in 1712 as a military engineering school.

Since 1955 it has been named after the famous Russian engineer Alexander Mozhaysky.

The Academy has been used to train GRU officers, including some of the accused Fancy Bear hackers such as Dutch man Alexey Morenets.

A second explosion also occurred at the Institute of Microbiology in Pushkin - a suburb of St Petersburg - which one Russian news network described as "small household incident."

One person is thought to have been injured in the second explosion.

The city's General Directorate of Emergency Situations refused to confirm the second explosion,

“We do not confirm the explosion. A check is underway,” a spokesperson said.

A law enforcement source told Tass news service that a laboratory technician dropped a flask containing chemicals and got burnt.

“It looks like this incident was taken for an explosion,” the source said.

It appears unlikely that the two incidents were linked.