A homemade bomb exploded Wednesday morning in a school playground in southern Russia where policemen had gathered to do physical exercises, killing four and wounding six, officials said.
The bomb - a 76 mm artillery shell with a radio-controlled detonator - exploded in a playground in the Dagestani town of Kizilyurt, said Anzhela Martirosova, a spokeswoman for the regional branch of the Interior Ministry.
The assailants have not been identified, she said.
Dagestan, a predominantly Muslim region that is home to many ethnic groups, has been plagued by shootings, bombings and other violence, some spilling over from neighboring Chechnya and other attacks stemming from local disputes and criminal activity.
Russian officials have repeatedly declared that Israeli aviation poses a threat to the Russian military in Syria.