Investigators believe that about 30kg of TNT equivalent explosives were used in the blast

Four people have been killed and six wounded in a car bomb explosion outside a cafe in the Russian region of Dagestan.

The attack is the deadliest to hit the Northern Caucasus region since Monday's suicide bombing on a Moscow airport that killed 35 people and been blamed on fighters from the overwhelmingly Muslim area.

The blast took place at just after 10:00pm local time (1900 GMT) on Wednesday outside the Karavan cafe on the outskirts of the town of Khasavyurt.

"Four people were killed, all of them civilians," a local interior ministry spokesman told the RIA Novosti news agency.

Six people were wounded, three of whom were hospitalised.

A criminal investigation has been opened into an "act of terror and illegal use of explosives", the Interfax news agency said, quoting the local branch of  the investigative committee.

Investigators believe that about 30kg of TNT equivalent explosives were used in the blast.

Attacks by fighters have claimed hundreds of lives in the Northern Caucasus over the last years as they wage a campaign against the authorities with the aim of imposing an Islamic state in the region.

Airport suspect

In a separate development, Russian media reports on Thursday said that investigators were searching for an ethnic Russian member of a North Caucasus group who vanished last year and is suspected of involvement in the attack on the Moscow airport.

Investigators have yet to publish any firm conclusions three days after the attack but unofficial reports have made a link with the group and a mysterious explosion in Moscow on December 31.

The Kommersant newspaper said the investigation was focusing on a man named Razdobudko from the Stavropol region, just north of the Caucasus mountains, who is suspected of belonging to a local armed Muslim group, Nogaisky Dzhamaat.

Suggesting that he could have been the suicide bomber, Kommersant said the authorities now believed that the attacker was most likely to have been a male ethnic Russian rather than from one of the Caucasus ethnic groups.

"He is not the only person suspected of involvement in the attacks," it quoted a security source as saying. "So it's not worth hurrying with conclusions."

Similar reports also appeared on other Russian news outlets, with a security source telling RIA Novosti that Razdobudko had vanished along with his wife in October last year.

The news agency said police were looking up to 10 suspects from Caucasus in connection with the bombing

Kommersant also reaffirmed previous reports the airport attack appears linked to a blast in Moscow late on December 31 where a suspected female suicide bomber is believed to have accidentally blown herself up.

Source: Agencies