Police at the site of an explosion in Dagestan's capital Makhachkala earlier this month (Reuters)

At least three people have reportedly been killed and more than 20 injured after two car bombs exploded in Dagestan's capital of Makhachkala.

"One car bomb was set off first but no one was hurt. The second explosion occurred about 15 minutes later when officers were at work," an official source told Interfax news agency.

The bombs went off outside the headquarters of the court bailiffs' service in the region's capital.

Initial reports, quoting Dagestan's top investigative agency, that set the death toll at eight have been later contradicted by other official sources saying only three people have been confirmed dead. The number remains disputed.

Russia's Dagestan Republic in the volatile Caucasus region is facing a decade long Islamic insurgency.

Earlier this month, three militant fighters were killed by police in woods about 60km (35 miles) south of Makhachkala. A few days earlier three people were killed in a separate incident, as a bomb went off outside a shopping centre in the city.

The issue of insurgency Dagetan rose up the news agenda in April when it emerged that Boston Marathon bombing suspect Tamerlan Tsarnaev spent six months in the region in 2012.

US detectives were trying to determine whether Tamerlan, 26, came into contact with notorious Islamist leader Gadzhimurad Dolgatov, aka "Robin Hood".

After returning to the US, Tamerlan posted several videos on his personal YouTube channel under a folder named "terrorism". One of these featured Dolgatov rallying against police co-operators.

'll kill you just like I'll kill them," Dolgatov is heard as saying. "If you side with the police you are helping Satan." Dolgatov was killed in a gun battle with Russian anti-terrorism police in December 2012.

Of Chechen origins, Tamerlan lived in in Dagestan with his family about a decade ago before moving to the US and investigators believe his return to the region in 2012 played an important role in his radicalization.

Police believe Tamerlan, 26, and his brother Dzhokhar, 19, may have been motivated by radical religious views to carry out the April 15 deadly attack that killed three people including an eight-year-old child and injured more than 260 in Boston.

Tamerlan died in a shootout with police, while his brother was apprehended after an intensive manhunt.

Dzhokhar has been charged with using a weapon of mass destruction to kill, and he could face the death penalty if convicted.