A bomb blast in Russia's southern Republic of Dagestan, has killed four police officers and wounded six others.

The explosion occurred Wednesday, as the officers were doing morning calisthenics on a school playground in the town of Kizilyurt.

Officials of the local branch of Russia's interior ministry say the bomb consisted of an artillery shell and a radio-controlled detonator.

Investigators have not identified the assailants. Dagestan borders Chechnya, where Russian troops have been battling separatist forces for more than a decade.