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 Nasra group, described as "obscure" by the BBC, posted a video online claiming it had carried out the explosions.
Al Nasra 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.
<em>Syrian mourners attend the mass funeral of 44 people killed in twin suicide bombings which targeted intelligence agency compounds at the Omayyad Mosque in Damascus on December 24, 2011 (LOUAI BESHARA/AFP/Getty Images)</em><br><br> Dec. 23, 2011 - Two car bombers blow themselves up near intelligence agency compounds in the capital Damascus, killing 44 people in the first suicide attacks since the uprising began in March.
<em>In this photo released by the Syrian official news agency SANA, an Arab league observer with the orange jacket, center, looks toward Syrian dead bodies who were killed during an explosion in Midan neighborhood in Damascus, Syria, on Friday Jan. 6, 2012. (AP Photo/SANA) </em><br><br> Jan. 6, 2012 - Two weeks later, another suicide explosion rips through a busy intersection and a police bus in the capital's Midan neighborhood, killing 26 people.
<em>In this photo released by the Syrian official news agency SANA, covered dead bodies are seen in front a damaged building at a security compound which was attacked by an explosion, in the northern city of Aleppo, Syria, on Friday Feb. 10, 2012. (AP Photo/SANA)</em><br><br> Feb. 10, 2012 - Two suicide car bombers strike in the northern city of Aleppo, Syria's largest, killing 28 people near a military intelligence building and the barracks of security forces.
<em>In this Friday, March 2, 2012, citizen journalism image provided by the Local Coordination Committees in Syria and accessed on Saturday, March 3, 2012, anti-Syrian regime protesters hold up banners during a demonstration, in Idlib, Syria. (AP Photo/Local Coordination Committees in Syria) </em><br><br> March 3, 2012 - A suicide car bomb explodes at a roundabout in Daraa, the birthplace of the uprising, killing at least two people in the first suicide attack in an opposition stronghold.
<em>In this photo released by the Syrian official news agency SANA, burnt and destroyed cars are seen near the aviation intelligence department, which was attacked by one of two explosions, in Damascus, Syria, on Saturday, March 17, 2012. (AP Photo/SANA)</em><br><br> March 17, 2012 - Three suicide bombings in Damascus kill at least 27 people, two of them near-simultaneous suicide car bomb attacks on heavily guarded intelligence and security buildings.
<em>A man reacts at a funeral of the victims killed during a suicide bombing attack in Damascus, Syria, Sunday, March 18, 2012. (AP Photo/Muzaffar Salman)</em><br><br> March 18, 2012 - An explosion hits near a government security building in Aleppo, killing one policeman and one woman.
<em>Syrian investigators, right, gather next to a damaged police bus that was attacked by an explosion in the Midan neighborhood of Damascus, Syria, on Friday April 27, 2012. (AP Photo/Bassem Tellawi)</em><br><br> April 27, 2012 - A suicide bomber in Damascus detonates an explosives belt near members of the security forces, killing at least nine people outside a mosque in the Midan neighborhood.
<em>In this photo released by the Syrian official news agency SANA, a Syrian man, left, stands in front of a building that was damaged after two bombs exploded near a military compound, in the city of Idlib, northwestern Syria, Monday, April 30, 2012. (AP Photo/SANA)</em><br><br> April 30, 2012 - Twin suicide explosions near daybreak strike close to a government security compound in the city of Idlib, killing at least nine people.
<em>People look at destroyed and damaged vehicles after an explosion on al-Thawra Street in Damascus, Syria, Saturday, May 5, 2012. (AP Photo/Bassem Tellawi)</em><br><br> May 5, 2012 - A bomb strikes a car wash in Aleppo, killing at least five people a day after government troops opened fire to break up large protests against a violent university raid.
<em>In this photo released by the Syrian official news agency SANA, flames and smoke raise from burning cars after two bombs exploded, at Qazaz neighborhood in Damascus, Syria, on Thursday May 10, 2012. (AP Photo/SANA)</em><br><br> May 10, 2012 - Two suicide blasts rip through Damascus' Qazaz neighborhood, killing 55 people in an attack that targeted a military intelligence building as employees were arriving at work.