A video posted online in the name of a shadowy militant group late Friday claimed responsibility for twin suicide bombings in the Syrian capital this week that killed 55 people. In the video, a group calling itself 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 distorted voice says, reading black text that rolls across a white screen while Islamic chanting plays in the background. 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 al-Qaida. 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. Their presence adds a wild card element to the Syria conflict that could further hamper international efforts to end it. Syrian soldiers check a burned truck in front of a damaged military intelligence building where two bombs exploded, at Qazaz neighborhood in Damascus, Syria, on Thursday May 10, 2012. Two strong explosions ripped through the Syrian capital Thursday, killing or wounding dozens of people and leaving scenes of carnage in the streets in an assault against a center of government power. (AP Photo/Bassem Tellawi) Close 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. But while foreign diplomats and the more than 100 U.N. observers in Syria can pressure the government and the opposition to stick the plan, they have no means of influencing shadowy Islamic militants. Friday's 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 on May 10, 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. The U.N. said weeks ago that more than 9,000 people have been killed.