MOSCOW, February 15 (RIA Novosti) - Six suspected militants possibly involved in a suicide car bombing at a police checkpoint in the Russian North Caucasus republic of Dagestan have been killed in a police operation, the Interior Ministry reported.

Four police officers were killed and six wounded in the attack that occurred in the northern Dagestani town of Khasavyurt on Thursday.

The blast left “a crater four meters in diameter and 1.5 meters deep,” the Investigative Committee reported in a statement. The explosives contained between 100 and 120 of TNT, according to Investigative Committee spokesman Vladimir Markin.

The police forces ambushed suspected militants linked to the attack not far from the blast scene. One of killed militants had probably organized the attack.

Some of the killed policemen had been posted to Dagestan from the Vologda Region, some 1,800 km north of Dagestan. The Vologda Region’s authorities announced a day of mourning on Friday.

The Investigative Committee has opened a criminal case on terrorism charges.

The Islamist insurgency, once confined largely to the republic of Chechnya, has spread across the North Caucasus in recent years. Attacks on security forces, police and civilians are reported regularly in the neighboring republics of Dagestan, Ingushetia and Kabardino-Balkaria.

Russian President Vladimir Putin said in mid-October that more than 300 terrorists were killed in the North Caucasus “in the past few months.” “Terrorism” in official Russian politics is used loosely and in preference to more neutral terms, such as “insurgency.”

Over a decade after the war against Islamist separatists in Chechnya ended, Russian security forces continue to fight militants in the volatile region.