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At least 18 people have been killed and dozens more wounded in an explosion at a college in Russian-annexed Crimea, BBC reports.

Officials said an "unidentified explosive device" detonated at the technical college in Kerch, where Russia has built a bridge between the peninsula and Russia.

Initial reports had suggested that the blast was some sort of gas explosion. But a Russian national guard official said the incident was a deliberate "terrorist act".

Sergei Melikov said the explosion had been caused by an improvised explosive device.

National guard units had reached the scene of the "act of terrorism"' he told Interfax.

A local official said most of the blast victims were students of the technical college, which is a vocational school for teenagers.

Victims have been taken to hospital, and a major emergency response operation launched.

Four military planes were ready to evacuate the wounded and military hospital facilities were ready to accept victims if necessary, Russian Defence Minister Sergei Shoigu said.

Investigators released a statement saying an explosive device filled with "metal objects" had detonated in the dining area.