An Orthodox priest is among the victims of apparently coordinated attacks on places of worship in Russia on Sunday. 

The attacks happened across the cities of Derbent and Makhachkala in the predominantly Muslim republic of Dagestan in the North Caucasus. 

A synagogue, churches and police posts were targeted, leaving 15 police officers and four civilians dead. 

The attackers, who have not been identified, used automatic weapons. Russian authorities said the attackers belong to "an international terrorist organization."

Sergei Melikov, the head of the Dagestan region, named the slain priest as Nikolai Kotelnikov and said he had worked at the church in Derbent for over 40 years. 

Three days of mourning have been declared in Dagestan, which neighbours Chechnya.

Three months ago, over 140 people were killed in a terrorist attack on Crocus City Hall in Moscow. ISIS affiliate ISIS-K claimed responsibility for that attack.