LATEST: Russia-imposed Crimean leader Sergei Aksyonov says the suspected attacker at a college in Kerch was a 22-year-old student or former student there and has killed himself.
He says the death toll is 18.
More to follow.
Russian authorities say a bomb filled with shrapnel exploded at a college in the Crimean city of Kerch, killing 13 people and injuring dozens in what investigators are treating as a terrorist act.
The blast ripped through the cafeteria at a polytechnic college in the city on the eastern end of the peninsula that Russia seized from Ukraine in 2014, Investigative Committee spokeswoman Svetlana Petrenko said.
State news agencies cited the Russian National Anti-Terrorist Committee (NAK) as saying it was caused by an unidentified explosive device, and Petrenko said the bomb was packed with metal objects.
"At the moment, we know of 13 dead and about 50 injured, most of them teenagers," Petrenko said, adding that casualty figures were being checked.
Students at colleges in Russia are usually about 14-17 years old.
Several news reports and a video purporting to show the distraught director of the college indicated that assailants fired guns in addition to setting off explosives, and depicted a scene of carnage at the college.
The incident is likely to increase tension between Russia and Ukraine, which Moscow has accused in the past of seeking to carry out acts of sabotage in Crimea.
The regional Emergency Situations Ministry declared a state of state of emergency in Crimea and said security was being increased. A three-day mourning period was announced by the Russian authorities who control the Black Sea peninsula.
In a video posted on the Internet, a woman identified as the director of the college, Olga Grebennikova, says there were "many corpses, many corpses of children" and calls it "a real terrorist act."
The woman, speaking on a mobile phone, says that shortly after she left the college for a meeting elsewhere, assailants attacked the school, throwing explosive devices and firing guns.
"They ran up to the second floor with automatic rifles -- I don't know with what -- and opened doors...and killed everyone they could find," she said. She said students and faculty were among the dead.
The authenticity of the video could not immediately be independently confirmed.
President Vladimir Putin's spokesman, Dmitry Peskov, said Russian authorities were considering terrorism as a possible motive.
Putin voiced condolences to victims and their loved ones and ordered the authorities to evacuate badly injured victims by air to "leading" hospitals in Moscow and other cities, Peskov said.
Some earlier reports had quoted officials saying it was a gas explosion, but a spokeswoman for the regional gas-distribution company said it had no contract to supply gas to the school.
Eyewitnesses said ambulances and other emergency vehicles rushed to the site after the blast, which some reports said took place in a cafeteria.
Kerch is the site of the western end of a newly built bridge linking Russia 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.
Russia sent troops to the peninsula, secured key buildings, and staged a referendum denounced as illegitimate by Ukraine, the United States, and a total of at least 100 countries.
Rights activists say Russian authorities have used the courts and law enforcement to conduct a campaign of reprisals against Crimeans who opposed the takeover.
Explosions in former Soviet republics are often caused by household cooking or heating gas, but Russia has also been hit by many terrorist bombings since the separatist wars in Chechnya in the 1990s and early 2000s.
The wars were followed by an Islamist insurgency in the North Caucasus region, east of Crimea, and insurgents have claimed responsibility or been blamed for most of the bombings and other attacks in Russia over the years.