Syrian rebels said they will start executing a group of about 48 Shiite pilgrims from Iran if Tehran and Damascus do not comply with their demands, according to a video posted this week on YouTube.
The pilgrims were abducted in August.
In the video, a rebel officer -- dressed in camouflage and standing in front of what appears to be a group of hostages -- addresses the Iranian and Syrian governments.
The officer says his group has tried to negotiate with Syrian authorities for the hostages' release in exchange for that of rebels held in regime prisons, but those efforts failed "because of the reluctance of both the Iranian and Syrian regimes."
Therefore, he continues, "Unless they start releasing our people from their prisons and cease the shelling of the innocent civilians in our cities and the ongoing random slaughter, within 48 hours, starting from the moment this statement is read, we inform you that for every martyr who gets killed by the Syrian regime, we will kill one of the Iranian hostages."
CNN cannot verify the authenticity of the video.
In August, Iran's Foreign Ministry said it was working through diplomatic channels to free the hostages.
At least 127 people were killed by Syrian security forces across the country Friday, including 28 who died in Damascus and its suburbs, according to the Local Coordination Committees for Syria, a network of opposition activists.
In other developments:
Rebels shoot down a helicopter gunship and a jet, government opponents say
Rebel fighters shot down a Syrian helicopter gunship over the town of Saqba, outside Damascus, the Syrian Observatory for Human Rights, a London-based opposition human rights group, said Friday. The helicopter was taking part in a bombardment in the eastern Ghouta area, the group said.
Social media videos posted by Syrian activists Friday showed a helicopter falling from the sky in the eastern Ghouta area. CNN cannot independently verify the authenticity of the videos.
Opposition fighters also reported shooting down a jet in Deir Ezzour province.
Shelling rages in some cities
Shelling continued in cities including Daraa, Homs, Aleppo and Hama on Friday, the LCC said.
In Homs, shelling resulted in a number of casualties, damaged homes and spread fires, the LCC said. Also, warplanes were firing on parts of the town of Marea, in northern Syria, the opposition group said. Marea is about 25 kilometers (16 miles) from Turkey.
The Syrian Observatory for Human Rights and Syrian state TV both reported clashes between rebel fighters and regime forces in Aleppo, near an air force Intelligence unit.
A mortar shell fired from Syria landed in an empty field near the village of Yayladagi, in Hatay province, Turkish state TV TRT said Friday. There were no casualties and the Turkish military in the area responded immediately, the channel said.
Erdogan warns Damascus against making a "deadly mistake"
Turkish Prime Minister Recep Tayyip Erdogan condemned Syria's shelling of a border town in Turkey on Wednesday, according to the semi-official Anadolu News Agency.