Islamists kill 15 Alawite and Druze civilians in Syria: activists

One other victim was a pro-government fighter, said the British-based monitoring group, which has a network of sources across Syria.

Syria's Sunni Muslim majority has largely joined the revolt against President Bashar al-Assad, who is an Alawite, while minority sects have largely stood behind him in the conflict that has killed more than 125,000 since it began in March 2011.

Many cities in Syria have become segregated along sectarian lines but Adra - strategically important as one of the rebels' only routes into Damascus - has mostly resisted that.

Witnesses, activists and Syrian state media accused the Islamic Front, an alliance of several large rebel groups, and the al Qaeda-linked Nusra Front of carrying out the attacks.

One woman in Adra, who said she was too frightened to say what religious minority she was from, told Reuters armed men from the Islamic Front and the Nusra Front entered her family's house around 3 a.m. (0100 GMT) on Thursday and took her son.

The militants said her son would be returned, but he was still missing on Thursday afternoon, she said.

Adra, about 20 km (12 miles) northeast of Damascus, has a population of about 100,000 including Alawites, Druze, Christians and Sunni Muslims.

Activists on social media said some residents had fled the city while others were hiding in their basements.

Several blamed the Army of Islam, an Islamic Front member group led by Zahran Alloush, and some put the death toll as high as 40, but these reports could not be independently verified.

"Zahran Alloush has committed a massacre," one activist based in the Damascus suburbs told Reuters.

State news agency SANA said the army was sending troops to "restore security" in the city.

"Terrorist groups belonging to the Nusra Front infiltrated the residential area of Adra in the suburbs of Damascus and attacked residents in their homes," it said, using its term for rebels fighting against Assad.