At least 13 people have died in a blast in a vocational school in the Russian-annexed peninsula Crimea, local media reported on Wednesday.
The explosion, which was initially reported as a gas explosion, injured at least 50 people in Kerch, a city of 147.000.
“At the moment, 13 are reported dead, 50 others were injured, most of them are teenagers. Reports on casualties are being verified,” Svetlana Petrenko, spokesperson for the Russian government, told TASS news agency.
The Russian government said it was treating the incident an attack, and bomb experts, armoured vehicles and personnel were sent to the school to secure and investigate the surrounding areas.
Local media reported that the explosion happened in the cafetaria, where an improvised explosive device (IED) allegedly went off.
The local Crimean government announced three days of mourning following the explosion and the Kremlin announced it would investigate the attack.
Russia annexed the peninsula in March 2014, after a referendum in which the population of Crimea chose to become part of Russia.
The annexation followed the overthrow of pro-Russian Ukrainian President Viktor Yanukovich after weeks of protests that descended into a violent conflict.
Later pro-Russian separatists took over parts of Donbass in southeastern Ukraine.
Since then, fighting in that region has killed more than 13,000 people, including about 3,000 civilians.
The so-called Minsk agreements – brokered by Germany and France and signed by Russia and Ukraine in 2015 – have slowed the conflict, but sporadic fighting continues.