Reports of local cease-fires and other short-term deals have become more common as Syria's three-year-old civil war drags on, but talks leading to prisoner exchanges still appear to be rare.
There was no immediate government comment. Calls to the Lebanon-based Greek Orthodox Patriarchate which oversees the convent went unanswered.
A spokesman for the rebel brigade al-Habib al-Moustafa said that so far government officials had refused the demand to release prisoners. The spokesman, who used the alias of Abu Nidal for security reasons, said a mediator was speaking to both parties. He said his group wasn't involved in negotiations, but was relaying information from other fighters.
The negotiations were also confirmed by a Syrian opposition activist who requested anonymity, as he was discussing talks conducted by other parties. He said the rebels were also demanding the release of imprisoned Saudi Arabian nationals captured while fighting for the opposition.
The activist said negotiations began immediately after the nuns were seized from their convent of Mar Takla in the village of Maloula, north of Damascus on Dec. 6 when rebels overran the area. At least another three women were also seized from the convent's orphanage. They were taken to the nearby rebel-held town of Yabroud, activists say.
The seized women appeared on a video days after their capture saying they were alive and well.
The rebel faction that released the video did not identify itself. No faction has announced that it has control of the women. Syrian opposition activists and Church officials have said the al Qaeda-linked group Jabhat al-Nusra, or the Nusra Front, is holding them.
The kidnapping of the women strengthened fears among Syria's minority Christians that al Qaeda-linked militants and other extremists, who are increasingly prominent in rebel ranks, are targeting them.
A priest and two bishops previously kidnapped by rebels remain missing, and extremists are accused of vandalizing churches in areas they have captured.
Christians and other minorities, like the Druse and Shiites, tend to support the government of President Bashar Assad, who comes from the country's minority Alawite sect. Syria's Sunni Muslim majority forms the backbone of the uprising against Assad.
Meanwhile, fighting continued on Saturday in an industrial area near Damascus where al Qaeda-linked rebels were earlier accused of killing Druse and Alawite men, women and children. The killings began on Wednesday, when rebels, mostly of the Nusra Front, overran the Adra industrial district and its neighbouring residential area northeast of Damascus, said state-run Syrian television and the British-based Syrian Observatory for Human Rights.
But rebel spokesman Abu Nidal and another rebel spokesman, Abu Yazan -- both based in the nearby area of Ghouta -- said the rebels only killed pro-government fighters in Adra and soldiers of the nearby Brigade 122 military base.
They acknowledged the fighters were Alawites and Druse but said they were killed because they were fighting for the government, not because of their sect. Syria's government has increasingly relied on militias, often drawn from minorities, to hold territory.
They said rebels were fighting in Adra to open up a road to Ghouta, which has been under siege for the past ten months by Syrian forces, and to cut a government road.