At least four police officers were killed on Wednesday when a bomb went off in the playground of a school in southern Russia, a local interior ministry spokeswoman said.
"There was an explosion at 7:40 am (0340 GMT) in the town of Kizilyurt on the territory of middle school number 7 during training exercises," the spokeswoman, Angela Martirosova, told AFP.
"Four people were killed, three were injured. They were all police officers," said Martirosova, adding that the blast occurred in the school playground.
Classes usually start later than the time of the blast in the southern Russian province of Dagestan, where Kizilyurt is located.
Attacks against security officers are frequent in Dagestan, which borders war-ravaged Chechnya. The attacks are often claimed by separatist rebels and Islamist extremists.