A series of attacks on churches and synagogues in Dagestan, a mainly Muslim region in Russia, has killed 20 people, including an Orthodox priest, after gunmen rampaged in two of the republic's most important cities.

Carrying automatic weapons, the gunmen burst into a Russian Orthodox Church and a synagogue in coordinated attacks on the ancient city of Derbent on the evening of June 23 and also on Makhachkal.

The attacker set fire to an icon at the church and killed a 66-year-old priest, Nikolai Kotelnikov, Reuters news agency reported.

At least 16 people were taken to hospitals with injuries, the BBC reported.

The attacks came just three months after fighters from the Islamic State group killed more than 140 people in a Moscow concert hall, the deadliest attack on Russia for almost 20 years, triggering renewed questions about security.

- Mainly Muslim republic

Dagestan, a predominantly Muslim republic in southern Russia that neighbors Chechnya, has declared three days of mourning.

The coordinated attacks targeted Derbent and Makhachkala on the Orthodox festival of Pentecost.

The head of the Republic of Dagestan, Sergei Melikov, later identified the priest as Father Nikolai Kotelnikov, who spent more than 40 years serving in Derbent.

In the city of Makhachkala, about 125 kilometers (75 miles) north on the Caspian Sea shore, attackers shot at a traffic police post and attacked a church.

Gun battles erupted around Makhachkala's Assumption Cathedral, and automatic gunfire was rife during the night, AFP reported.

TV footage showed residents running for cover as plumes of smoke rose above the city.

Some footage posted on social media showed people wearing dark clothes shooting at police cars before an emergency service convoy of vehicles arrived.

Derbent is also home to an ancient Jewish community, and gunmen attacked a synagogue and a church, setting them on fire.

Dagestan has, in the past, faced Islamist attacks.

Although the latest assailants have not been officially identified, Russian media reported that among the gunmen were two sons of the head of the Sergokalinsky district near Makhachkala - Magomed Omarov, Osman, and Adil. Police detained Omarov.

However, in a video post on Telegram, the republic's head, Sergei Melikov, suggested Ukraine had been involved in the attack and that Dagestan was now directly involved in Russia's war in Ukraine, according to the BBC..

"We understand who is behind the organization of the terrorist attacks and what goal they pursued," Melikov said.

The head of the Russian State Duma's international affairs committee, Leonid Slutsky, advocated similar claims, saying that the Dagestan attacks and a missile strike that killed four people in Russian-occupied Sevastopol on June 22 "could not be a coincidence."

"These tragic events, I am sure, were orchestrated from abroad and are aimed at sowing panic and dividing the Russian people," Slutsky said.

Russian authorities blamed the attack on the Crocus City Hall near Moscow in March, which left 147 dead, on Ukraine and the West, even though the Islamic State group claimed to have carried it out.