(CNN) -- 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 on YouTube this week.
The pilgrims were abducted in August.
In the video, a rebel officer -- dressed in camouflage and standing in front of what appeared to be a group of hostages -- addressed the Iranian and Syrian governments.
The officer said his group had 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 continued, "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, such as the United Nations, to free the hostages.
At least 55 people were killed by Syrian security forces across the country Friday, including 17 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
A Syrian helicopter gunship was shot down by rebel fighters over the town of Saqba, in the Damascus countryside, the Syrian Observatory for Human Rights, a London-based opposition human rights group, said Friday. The helicopter was taking part in the bombardment in the eastern Ghouta area, the group said.
Earlier, the group said that a warplane had been shot down but corrected its statement by saying it was a helicopter. 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 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 the border with Turkey.
The shelling in Homs left many people badly injured, the LCC said.
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 fell 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.
U.N. condemns shelling in Turkey
The U.N. Security Council on Thursday condemned Syria's shelling of a border town in Turkey and appealed for restraint from both countries.
"The members of the Security Council underscored that this incident highlighted the grave impact the crisis in Syria has on the security of its neighbors and on regional peace and security," said Gert Rosenthal, ambassador from Guatemala, speaking in his role as council president.
The shelling of the Turkish town of Akcakale resulted in the deaths of five civilians -- all of them women and children -- and a number of injuries, Rosenthal said.
Turkey responded to the Wednesday incident by shelling a Syrian military position.
"Turkey has no interest in a war with Syria," Ibrahim Kalin, foreign policy adviser to Prime Minister Recep Tayyip Erdogan, said Thursday in a posting on Twitter. "But Turkey is capable of protecting its borders and will retaliate when necessary."
Moscow mulls diplomatic moves
Russia hopes the U.N.-Arab League envoy to Syria, Lakhdar Brahimi, will visit Moscow soon, perhaps this month, Deputy Foreign Minister Mikhail Bogdanov told the state-run Itar-Tass news agency Friday.
"We believe the sooner we will take active politico-diplomatic steps in this direction, the sooner the bloodshed is ceased," he is quoted as saying.
Moscow is a historic backer of President Bashar al-Assad's regime and has blocked previous U.N. Security Council resolutions on Syria.
Meanwhile, Russian Deputy Minister of Foreign Affairs Gennady Gatilov said on Twitter: "Everyone must be cautious in the #Syrian situation. Cross-border incidents resulting in escalation of the conflict are unacceptable."