However, the Investigative Committee, the state body that investigates major crimes, said later that it was re-classifying the case from terrorism to mass murder.
Military units were deployed around the college.
"In Kerch, an unidentified explosive device exploded", a spokesman for the National Anti-Terrorism Committee was quoted as saying. Officials at first reported a gas explosion, then said an explosive device had ripped through the college canteen during lunchtime in a suspected terrorist attack.
Witnesses report that schools and nurseries are being evacuated across Crimea following the blast, and the school is being inspected for other possible bombs.
The statements came after hours of rapidly shifting explanations by Russian officials as to what exactly happened at the school. He did not elaborate.
Dmitry Peskov, spokesman for Russian President Vladimir Putin, told reporters that officials are investigating. One witness told local television that armed gunmen were also involved in the attack.
Another student, Semyon Gavrilov, told the Komsomolskaya Pravda newspaper that he saw "a young man with a gun walking around shooting everybody", and how he himself had survived by hiding in a classroom.
Speaking to local media, the head of the college Olga Grebennikova said someone blew themselves up inside the college, leaving "the bodies of children everywhere".
Numerous victims from Wednesday's attacks were teenage students who suffered shrapnel and bullet wounds.
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