A video posted online in the name of a shadowy militant group has claimed responsibility for twin suicide bombings in the Syrian capital last week that killed 55 people.
In the video posted late Friday, the Al-Nusra Front says the bombing was in response to attacks on residential areas by the regime of President Bashar Assad.
"We fulfilled our promise to respond with strikes and explosions," a voice says.
The Al-Nusra Front has claimed past attacks through statements posted on militant websites. Little is known about the group, although Western intelligence officials say it could be a front for an al-Qaida branch operating in Iraq.
Western officials say there is little doubt that Islamist extremists, some associated with al-Qaida, have made inroads in Syria as instability has spread since the start of the uprising against President Bashar Assad 14 months ago.
But much remains unclear about their numbers, influence and activities inside Syria.
Their presence adds a wild card element to the Syria conflict that could further hamper international efforts to end it.
World powers are backing a peace plan presented by international envoy Kofi Annan that calls for a cease-fire to allow for talks on a political solution to the conflict.
A shipment of supplies for U.N. observers monitoring the truce arrived in Damascus. It included six armored cars, the first of 25 to arrive Saturday for use by the 105 military observers and 45 support staff currently in the country.
But while foreign diplomats and observers can pressure the government and the opposition to stick to the cease-fire, they have no means of influencing shadowy Islamic militants.
The video said the Damascus attack was in response to Syrian government attacks on residential areas. "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," it says.
The video, which says it was made Thursday, the day of the Damascus bombings, also strikes a sectarian tone, calling for the protection of Sunni Muslims and threatening revenge against Alawites, the Shiite offshoot to which Assad and many members of his security services belong.
"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 says.
It also advises Sunnis to avoid security offices -- a veiled threat of future bombings.
Thursday's twin blasts in Damascus were the fifth in a string of major attacks in Syrian cities that have clouded the picture of a fight between the opposition and the regime. It was the deadliest yet, in part because it happened on a key thoroughfare during rush hour, while previous bombings were on weekends.
Syria's uprising started in March 2011 with mostly peaceful protests inspired by successful revolts elsewhere calling for political reform. The Syrian government responded with a brutal crackdown, prompting many in the opposition to take up arms to defend themselves and attack government troops.
The U.N. said weeks ago that more than 9,000 people have been killed. Hundreds more have died since.