Six people were killed and at least 50 wounded when three car bombs exploded in the capital of the Muslim Dagestan region in Russia's North Caucasus, an Interior Ministry source and Islamist rebels said Thursday.

The first car detonated in Makhachkala on Wednesday at 7:30 p.m. (1530 GMT), killing at least four people and injuring five passers-by, said the source, speaking on condition of anonymity.

Two more car bombs exploded after midnight, killing one policeman and a civilian, the source told Reuters.

The debris of two cars lay crumpled on a main thoroughfare where blood was pooled near a row of shattered and blackened stores and cafes, state TV showed.

At least 50 people were injured in the attacks, which took aim at the regional office of the traffic police as well as the Interior Ministry, rebels said on vdagestan.com, which represents the Dagestani section of the insurgency.

Rebels are fighting for a separate Islamic state in the mainly Muslim North Caucasus along Russia's southern frontier in an insurgency underpinned by two post-Soviet wars in Chechnya, poverty and anger at heavy-handed tactics used by police.

Russian President Dmitry Medvedev has named the insurgency the country's top security threat in the year before the March 2012 presidential election.

The insurgency took responsibility for a suicide bombing at Moscow's Domodedovo airport that killed 37 people in January, as well as twin metro attacks last year that killed 40.

(Writing by Amie Ferris-Rotman; Editing by Robert Woodward)

Dmitry Medvedev Russia Dmitry Medvedev Terrorism