At least 10 people have died and many more wounded after a explosion at a college in Crimea.
However, Kremlin spokesman Dmitry Peskov told reporters that terrorism was among the versions being studied, and that President Vladimir Putin had ordered security services to investigate.
Sergei Melikov, a deputy chief of the Russian National Guard, said the explosive device was homemade.
Home to some 147,000 people, Kerch is considered to be one of the most ancient cities in Crimea.
Russian officials said it may have been a terror attack.
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Reuters news agency said the schools and pre-schools were being evacuated in the city after the blast.
Kerch is the site of the western end of a newly built bridge linking Russian Federation to Crimea, which it took over by force after Moscow-friendly Ukrainian President Viktor Yanukovych was pushed from power by pro-European protests in Kyiv.
A local official said most of the blast victims were students of the technical college, which is a vocational school for teenagers. Putin offered his condolences to the families of the victims. Earlier, emergency officials said the explosion was believed to have been caused by a gas canister explosion.
Russia's National Guard has called the explosion an act of terrorism, while the investigative committee has opened a terrorism probe.
The head of Crimea, Sergei Aksyonov, and Russia's Health Minister Veronika Skvortsova headed to the area to coordinate assistance to those injured. Located in the easternmost tip of Crimea's Kerch peninsula, the area is connected to mainland Russian Federation by the newly-built massive Crimean bridge.