A video posted online in the name of an Islamist group, al-Nusra Front, says 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.
Opposition activists have accused the regime of orchestrating the explosions.
The al-Nusra Front 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.
The video also warns Sunnis against "living near security buildings and dens of the regime or passing near them".
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.
And the fact that al-Nusra says it shares their goal of overthrowing President Assad may make some countries which support the opposition feel uneasy about the possibility of al-Qaeda infiltration, our correspondent adds.
Meanwhile, the funerals of some of those killed in Thursday's blasts have been taking place in the city.
A government minister taking part described the bombings as a "terrorist escalation".
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.