Their departure coincided with an assault by Assad's forces backed by tanks and helicopters to try to retake the Damascus suburb of Mouadamiya from opposition fighters, activists said. 

Twelve people, including three rebels, were killed, according to the opposition Syrian Observatory for Human Rights, a British-based group with many contacts in Syria.

  Control of Mouadamiya has changed hands several times, as have many other places in a war with no clear frontlines. 

"Our mission failed because the two sides did not abide by their commitments," said one uniformed observer, who declined to be named, before seven U.N. cars left a Damascus hotel carrying some of the last members of a mission once 300 strong.

  The unarmed monitors suspended operations in June after coming under fire and most have already departed, leaving a small liaison office in the Syrian capital in case a chance for a political settlement to the bloodshed ever emerges. 

Battling a 17-month-old uprising against his family's 42-year rule, Assad has used fighter jets and helicopter gunships to pound rebel strongholds. Insurgents have stepped up their attacks, hitting tanks, military convoys and security buildings.