Gunmen have attacked multiple facilities in Russia's southern republic of Dagestan. Russian investigative authorities say four civilians and 15 police officers were killed. They are investigating the case as terrorist attacks.

Media reports say the gunmen attacked two Russian Orthodox churches and two synagogues, and also police officers in the cities of Makhachkala and Derbent on Sunday.

A Jewish organization says the attackers set fire to a synagogue in Derbent. Video footage from the site shows a burned building.

Russia's National Antiterrorism Committee says five assailants were killed.

The republic of Dagestan announced three days of mourning from Monday for the victims of the attacks.

No group has yet claimed responsibility for the attacks.

Sergei Melikov, head of the republic, said the attacks had been prepared with support, including from abroad.

The Tass news agency quoted Russian security sources as saying the gunmen were adherents of an international terrorist organization.