At least three people have been killed and 12 more injured in back-to-back car bombings in the semi-autonomous Russian republic of Dagestan, Interfax news agency reports citing authorities.

The first bomb detonated on Monday in front of the Justice Ministry in the regional capital of Makhachkala.

As police descended upon the scene, a second car bomb was detonated by remote control, authorities said.

Most of the wounded and the dead were caught by the second explosion, the investigators said.

Police agencies gave differing death tolls, with the number ranging from three to eight. About 30 people were believed injured.

Images transmitted from the attack site depicted heavy damage.

Car bombs, suicide bombings and firefights are common in Dagestan, at the centre of an insurgency rooted in two post-Soviet wars against separatist rebels in neighbouring Chechnya.

Conflicting figures

Investigators initially said eight people were killed on Monday by the successive blasts, but Russian law enforcement officials later revised the figure down.

Twisted wreckage of a car could be seen near the building, which was cordoned off by police.

The main suspect in the Boston Marathon bombings in the US, Tamerlan Tsarnaev, lived in Dagestan with his family about a decade ago and visited the region last year.

The visit by Tsarnaev, who was shot dead by US police after the April 15 bombings that killed three people and wounded 264 others, is being scrutinised by US investigators for signs of ties with fighters.

Russian President Vladimir Putin has ordered law enforcement authorities to ensure fighters do not attack the 2014 Winter Olympics next February in the Black Sea resort city of Sochi, which is close to the North Caucasus.

Fighters in the North Caucasus have often sought to increase casualties by setting off an initial blast to attract law enforcement officers and then detonating a second bomb.

Dagestan, an ethnically mixed, mostly Muslim region between Chechnya and the Caspian Sea, has become the most violent province in the North Caucasus, where armed groups say they are fighting to carve out an Islamic state out of southern Russia.

At least 405 people were killed in Dagestan in violence linked to the insurgency last year, according to the Caucasian Knot website, which tracks developments in the region.

Putin launched the second war in Chechnya as prime minister in 1999 and likes to take credit for preventing the region from splitting from Russia. But his 13 years in power have been marred by deadly attacks claimed by or blamed on the fighters.