Thursday's blasts killed 55 and caused extensive damage to nearby buildings

A video posted online by an obscure Islamist group, al-Nusra, has said it carried out two bomb attacks in the Syrian capital Damascus on Thursday.

The attacks took place near a military intelligence building during the morning rush hour, killing 55 people.

Authorities had blamed "foreign-backed terrorists" for the attacks.

The al-Nusra group emerged in January and has said it was behind previous attacks, including one in March on a police HQ and airforce Intelligence.

The video says the bombings were in response to attacks on civilian areas by forces loyal to President Bashar al-Assad.

"We fulfilled our promise to respond with strikes and explosions," a distorted voice says in the video, according to the Associated Press.

"We tell this regime: Stop your massacres against the Sunni people. If not, you will bear the sin of the Alawites," the video continues, referring to the offshoot of Shia Islam to which Mr Assad and many of the ruling elite belong.

BBC security correspondent Frank Gardner says al-Nusra refers to its fighters as "mujahideen of Sham [Syria] in the arena of jihad" and there are suspicions it may have links to al-Qaeda.

The tactics used in the Damascus attacks are similar to attacks by al-Qaeda in Iraq in recent years, the BBC's Jonathan Head in Istanbul reports.

Meanwhile, the funerals of some of those killed in Thursday's blasts have been taking place in the city.

Anti-government activists had accused the regime of orchestrating the explosions in order to discredit the opposition.

Violence has been continuing across the country despite a ceasefire being monitored by a team of UN observers.

The UN estimates at least 9,000 people have died since pro-democracy protests began in March 2011.