The 13 nuns and three maids were kidnapped from the famed Christian hamlet of Maaloula last year and transported to the nearby town of Yabroud, a militant stronghold the army is fighting to recapture.

"I was in regular contact with them, but since yesterday we've lost contact," a source involved in negotiations said.

"It is very likely that they have been transferred to the region between Yabroud and the Lebanese border" nearby, the source said.

"Contacts are underway to try to ensure their safety."

The source said the kidnappers were from a group belonging to the Al-Qaeda representative Al-Nusra Front, led by a man called Abu Malek al-Kuwaiti.

Syria sank into war in March 2011 when pro-reform protests turned into a massive insurgency following the intervention of Western and regional states.

The unrest, which took in terrorist groups from across Europe, the Middle East and North Africa, has transpired as one of the bloodiest conflicts in recent history.

SHI/SHI