Gunmen in Russia's North Caucasus republic of Dagestan have killed six police officers in a series of attacks, security officials say.

Twelve other officers were also wounded in the attacks.

A synagogue, two churches and a police checkpoint were targeted in the cities of Derbent and Makhachkala.

A Russian Orthodox priest has also been killed and at least two attackers shot dead, state media report, quoting the local Interior Ministry.

The hunt for more gunmen is still ongoing.

The assailants have not been identified, but Dagestan has in the past been the scene of Islamist attacks.

Footage on social media shows buildings on fire in both cities.

In another video, people wearing dark clothes are seen shooting at police cars, before a convoy of emergency service vehicles, sirens blaring, arrive at the scene in Makhachkala, Dagestan's largest city.

An unofficial channel on the Telegram messaging app, Mash, said gunmen were barricaded in a building in Derbent.

Russia's Investigative Committee said it had launched an investigation over "acts of terror", AFP news agency reported.

In April Russia's FSB security service arrested four people in Dagestan suspected of involvement in the attack on Moscow's Crocus City Hall venue the previous month.

More than 140 people were killed in that attack, which was claimed by the Islamic State group.