At least 28,000 Syrians have disappeared since the protests against President Bashar Assad began 18 months ago, human rights groups have told the BBC.

Activists working in Syria said they have the names of 18,000 people missing and that they know of another 10,000 cases of people who have disappeared after being abducted by soldiers or militia.  

The online activist group Avaaz collected testimony from Syrians who said their family members have been abducted by Assad forces.

The campaign director at Avaaz, Alice Jay, said that Syrians are "plucked off the street by security forces and paramilitaries and being disappeared into torture cells."

"The panic of not knowing whether your husband or child is alive breeds such fear that it silences dissent," Jay told the BBC.

The Syrian Network for Human Rights has said that about 28,000 people have disappeared since anti-government protests began.  The Syrian human rights organization Sawasya, meanwhile, estimated that the figure is even higher.

"According to information given to us by our contacts in villages across Syria, we think there could be as many as 80,000 forcibly disappeared people," he told the BBC. "People are being snatched at night, on the street and when no-one is looking."

On Wednesday, the Syrian government said UN peace envoy Lakhdar Brahimi’s call for a holiday cease-fire would likely fail because the rebels fighting to topple Assad’s regime have no unified leadership to agree to it.

Brahimi, had asked Iranian officials to help broker a truce during the Muslim holiday Eid al-Adha, which falls later this month.

But Syria’s state-run Al-Thawra newspaper, a government mouthpiece, said Wednesday that the biggest obstacle to the truce was the lack of an authority to sign for the rebels.

All international efforts to end Syria’s civil war to date have failed. Both rebels and government forces have disregarded previous cease-fires, and the scores of rebel units fighting to topple the regime have no unified leadership. Many don’t communicate with each other.