The authorities in Russia's Republic of Dagestan declared three days of mourning after a series of attacks, Dagestan's head Sergei Melikov announced on Monday.

From Monday to Wednesday, flags will be lowered to half-staff throughout the republic and all entertainment events will be canceled.

Melikov had earlier said that over 15 police officers and several civilians were killed by armed militants in attacks at two orthodox churches, a synagogue and a traffic police post in the cities of Derbent and Makhachkala on Sunday.

Six gunmen were later killed in a counterterrorist operation, Melikov said, adding that the situation was under control.

There was no immediate claim of responsibility for the attacks.

(With input from agencies)