The Syrian government is negotiating with rebels to release 12 nuns seized earlier this month from a convent north of Damascus, two activists said.

The rebels are demanding that hundreds of imprisoned women activists be freed in exchange for the nuns, they added.

Reports of local ceasefires and other short-term deals have become more common as Syria's three-year-old civil war continues, but talks leading to prisoner exchanges still appear to be rare.

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 said a mediator was speaking to both parties.

He said his group was not involved in negotiations, but was relaying information from other fighters.

The negotiations were also confirmed by a Syrian opposition activist. 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 December 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-Qaida-linked group Jabhat al-Nusra, or the Nusra Front, is holding them.

There was no immediate comment from the government or the Lebanon-based Greek Orthodox Patriarchate, which oversees the convent.

The kidnapping of the women strengthened fears among Syria's minority Christians that al Qaida-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 vandalising churches in areas they have captured.