Gunmen attacked synagogues and churches in two cities in southern Russia on Sunday, killing multiple police officers and a priest, in an apparently coordinated assault that underscored Russia’s vulnerability to extremist violence.

Officials said six of the gunmen were killed after shootouts in the two cities, Makhachkala and Derbent, in the predominantly Muslim region of Dagestan on the Caspian Sea. Wielding rifles and Molotov cocktails, they attacked a synagogue and a church in each of the two cities, according to the authorities and religious organizations.

Sergei Melikov, Dagestan’s governor, described the attack as the latest assault “on our fraternity, on our multiethnic unity.”

The precise death toll was not immediately clear. Mr. Melikov said that “more than 15 police officers fell victim to today’s terrorist attack,” without specifying how many of them were killed and how many were wounded.

The motives and identities of the gunmen were also unclear, and there was no claim of responsibility for the attack. Russia’s Investigative Committee, an analogue to the F.B.I., said it had begun a terrorism investigation.

The attack was the latest outburst of apparent extremist violence inside Russia as the country fights its war against neighboring Ukraine. Four gunmen killed 145 people at a Moscow concert hall in March in an attack for which the Islamic State claimed responsibility. And in Dagestan last October, an antisemitic mob stormed a plane arriving from Tel Aviv.