At least 19 people - students and others were killed along with dozens more being injured in a mass shooting incident carried out by another student in a college in Russia-annexed region of Crimea.

The suspected perpetrator also "shot himself dead" after the attack, said Crimean government head Sergey Aksyonov.

At least 20 people were killed and 50 injured in the gunfire and the blast, which officials told Reuters went off in the school cafeteria. 12 of whom were in an extremely serious condition.

The suspect was identified as 18-year-old Vladislav Roslyakov, a fourth-year student at the school, according to a spokeswoman for Russia's Investigative Committee. Euronews reported some people died as a result of a blast from an improvised explosive device filled with metal objects placed in the school's dining room.

A few hours later the Investigative Committee said the victims were killed by gunshots. "The gunman had a hunting gun", Crimean official Igor Mikhailichenko said.

The criminal case was initially opened over an act of terrorism, but was later reclassified as murder. "It was a real act of terrorism. It's a mass killing committed by one scumbag", Aksyonov said earlier on state television.

The committee said all the victims died of gunshot wounds.

Eyewitnesses at the school, attended by children between the ages of 14 and 18, said gunmen fired into the air before the explosion, Russian media reported. A three-day mourning will start in Crimea on Thursday.

Kremlin spokesman Dmitry Peskov said President Vladimir Putin had been informed of the "tragic news" from Kerch and that he "expresses deep condolences to those who lost relatives and loved ones".

"It's already clear that it's a crime, the motive and possible versions for this tragedy are being carefully examined", Putin said. There was also an explosion at the scene but Russian authorities are yet to confirm the source. A student at the college who asked not to give his name told AFP that "I was in a class when I heard shooting on the first floor".

"When we all ran out into the corridor, there were others running and shouting that some guy with a machine gun was randomly shooting everyone in turn".

The Investigative Committee said an explosive device rigged with shrapnel went off in the lunchroom and Sergei Melikov, a deputy chief of the Russian National Guard, said it was homemade.

An unnamed witness who worked nearby said in a video on Kerch.tv website victims "were taken away in public transport, in minibuses and buses, with 2 or 3 people in each ambulance".

Reflecting the official confusion, Russian President Vladimir Putin said the victims were killed by an explosion just as the Investigative Committee was announcing they were fatally shot. He said he looked out and saw a young man with a rifle shooting at people.

In April, another student stabbed a teacher and a fellow student in the Urals Mountains and then set fire to a classroom.

Attacks by disaffected teenagers at schools and colleges have hit the headlines recently in Russian Federation.