UNITED NATIONS -- Armed fighters linked to the Syrian opposition detained 21 U.N. peacekeepers Wednesday in the increasingly volatile zone separating Israeli and Syrian troops on the Golan Heights, a new escalation in the spillover of Syria's civil war.
The U.N. Security Council demanded their immediate and unconditional release.
The capture comes a week after the announcement that a member of the peacekeeping force is missing. The force, known as UNDOF, was established in 1974 following the 1973 Yom Kippur war to monitor the disengagement of Israeli and Syrian forces and maintain a cease-fire.
Israeli officials have grown increasingly jittery as the Syrian war moves closer to Israel. There have been several instances in which stray fire has landed in the Israeli-controlled Golan Heights, and Israel is concerned that Syrian weapons could fall into the hands of hostile groups and be used against Israel. Israel captured the Golan Heights from Syria in 1967 and Syria wants the land returned in exchange for peace.
Russia's U.N. Ambassador Vitaly Churkin, the current Security Council president, said talks are under way between U.N. officials from the peacekeeping force and the captors.
U.N. peacekeeping chief Herve Ladsous, who briefed the council behind closed doors, identified the captors as being from a group associated with the Syrian armed opposition, Churkin said.
"There was no fighting, according to his briefing to us," Churkin said. "My understanding is that they took over the trucks in which the UNDOF personnel was moving around."
Churkin said the capture of the peacekeepers "is particularly unacceptable and bizarre" because the UNDOF peacekeepers are unarmed and their mission has nothing to do with Syria's internal conflict.
"They are there on a completely different mission so there is no reason at all under any circumstances, any kind of sick imagination to try to harm those people," he said.
U.N. Secretary-General Ban Ki-moon also condemned the capture of the 21 peacekeepers, U.N. deputy spokesman Eduardo del Buey said.
Del Buey said the U.N. observers were on a regular supply mission when they were stopped by approximately 30 armed fighters near an observation post that was damaged in heavy combat last weekend and had been evacuated.
A video posted online by activists showed a group of armed rebels standing around at least three white U.N. vehicles with the words UNDOF on them, allegedly in the village of Jamlah in Daraa province.
The video, circulated by the Syrian Observatory for Human Rights, said the peacekeepers being held by the rebels are 20 Filipinos.
It accuses the peacekeepers of assisting the Syrian regime to redeploy in an area near the Golan that the fighters had seized a few days ago in battles that left 11 fighters and 19 regime forces dead.
A man identified as Abu Qaed al-Faleh, spokesman for the Martyrs of Yarmouk Brigades, announced the group is holding the peacekeepers until Syrian President Bashar Assad's forces withdraw from Jamlah.
"They will not be released until after Bashar Assad's forces withdraw from the village of Jamlah bordering Israel," the man said.
Churkin urged countries with influence on the Syrian opposition to use it to help free the peacekeepers. He did not name any countries but Qatar, Saudi Arabia and Turkey are known to have been providing military aid to some Syrian rebel groups.
The international community has been divided in its response to Syria's conflict.
The United States and other countries have supported Syria's political opposition but have been reluctant to send weapons partly because of fears they may fall into the hands of extremists who have been gaining influence among the rebels. The Obama administration, however, announced last week that it would, for the first time, provide non-lethal aid directly to the rebels.
Russia and China, meanwhile, have continued to back Assad's regime.
Human Right Watch, meanwhile, is investigating whether the same rebels linked to seizing the peacekeepers were involved in the executions of captured regime soldiers in another incident around Jamlah several days ago. The rights group began the investigation after receiving one video apparently showing the capture of the Syrian soldiers and a second video showing bodies in the same area, Peter Bouckaert of Human Rights Watch said.
"We were just starting to investigate this today ... when we learned about the incident with the UN peacekeepers," he said.
Asked about why the rebels might be holding the U.N. peacekeepers, he said: "This seems to be a rather inexperienced group. It shows the desperation that many people, including armed groups, around Syria feel about protecting the civilians in their own villages."
Ban has warned of escalating military activity along the Israeli-Syrian border as a result of the intensifying Syrian conflict, which began in 2011 and has cost more than 70,000 lives.
In December, Ban accused the Syrian government of serious violations of the 1974 separation agreement and called on both countries to halt firing across the cease-fire line. He cited numerous clashes between Syrian security forces and opposition fighters in the disengagement zone.
In response, he said, UNDOF has adopted a number of security measures.
Associated Press writers Karin Laub and Zeina Karam in Beirut, Lebanon, contributed to this report.