A group of Islamist extremists have claimed responsibility for a twin bombing in Syria's capital Damascus on Thursday, which killed 55 people and injured more than 370 others.
The al Nusra group, described as "obscure" by the BBC, posted a video online claiming it had carried out the explosions.
Al Nusra first emerged in January and has previously said it carried out other attacks, including one in March at a police headquarters.
"We fulfilled our promise to respond with strikes and explosions," a distorted voice says in the video which was made on 10 May, the day of the bombing.
"We promised the regime in our last declaration to respond to its killing of families, women, children and old men in a number of Syrian provinces, and here we kept our promise."
The Associated Press said that the video showed black writing rolling on a white screen while Islamic chanting played in the background.
"We tell this regime: Stop your massacres against the Sunni people. If not, you will bear the sin of the Alawites. What is coming will be more calamitous, God willing," it said.
According to the AP the group could be a front for an al-Qaeda branch operating in Syria.
The Syrian government blamed "armed terrorist" for the blasts, which were also condemned by the UN observer team currently in the country to observe a fragile ceasefire.
Activists also reported heavy shelling in cities including Homs throughout the week, blaming government forces.
The Syrian Observatory For Human Rights said more than 12,000 people have been killed in Syria since the start of the uprising there in March 2011.