BEIRUT: Clashes between Syrian troops and rebels flared close to Israeli-controlled territory Thursday, a day after rebels detained 21 U.N. peacekeepers in the area in another destabilizing twist to the country's two-year-old conflict.

The abduction Wednesday of nearly two dozen Filipino peacekeepers marked the first time since U.N. troops began patrolling an Israeli-Syrian armistice line in the Golan Heights nearly 40 years ago that U.N. forces have encountered trouble during their mission, said Timor Goksel, a former United Nations official in the region.

The targeting of the peacekeepers was likely to heighten Israeli jitters about the Syrian civil war upsetting the delicate balance along the frontier between the two countries.

Israel captured Syria's Golan Heights in the 1967 Mideast war, and a U.N. monitoring force, UNDOF, was sent in 1974, a year after another Mideast war, to enforce an armistice deal between Syria and Israel.

On Thursday, Syrian troops battled rebel fighters near the Golan Heights, in the southern Syrian province of Daraa, said Rami Abdul-Rahman, the director of the Britain-based Syrian Observatory for Human Rights activist group. He said the fighting was concentrated on the outskirts of the Syrian village of Jamlah, about one kilometer (mile) from Israeli-controlled territory.

On Wednesday, rebels detained 21 peacekeepers, all from the Philippines, near Jamlah.

In an amateur video posted online, a man identified as a spokesman for the Martyrs of Yarmouk Brigades said his group will hold the peacekeepers until Syrian President Bashar Assad's forces withdraw from Jamlah.

The Yarmouk Brigades said in a statement on its Facebook page on Thursday that Assad's troops are shelling Jamlah, and warned that the army will be responsible if the peacekeepers in rebel custody are harmed.

The Philippine government said Thursday that the peacekeepers were unharmed and were being treated well. Foreign Affairs Department spokesman Raul Hernandez said the U.N. force commander in the area is negotiating with the leader of the rebel group.

Philippine President Benigno Aquino III said the commander told him to expect the peacekeepers to be released within 24 hours, with negotiations progressing well.

The U.N. Security Council has demanded their immediate and unconditional release.

It was not immediately clear if UNDOF will keep operating in Syria even if the incident is resolved peacefully.

A man who answered the phone at UNDOF's office in Damascus said he was not authorized to give statements referring questions to the U.N. in New York.

Goksel, the former U.N. official in the region, described the members of the peacekeeping force as "a soft target." He said the group is based in Damascus, but staffs observation posts along the armistice line, and travels between the Syrian capital and the frontier to deliver supplies and rotate monitors.

"They were never challenged by anybody in Syria until now," Goksel said.

The Yarmouk Brigades, one of scores of groups fighting Assad's troops, was formed a year ago and most of its fighters appear to be young Syrians from poor areas in the south, said Observatory director Abdul-Rahman.

In a statement Thursday, the Western-backed opposition group, the Syrian National Coalition, said its representatives are in contact with rebels in the Jamlah area "to let the peacekeepers go." The statement denied that seizing the peacekeepers amounted to kidnapping, saying the peacekeepers were taken in a "preventive security measure."

Rebel groups tend to operate independently, despite attempts in December to form a unified military command, and it's not clear whether the local rebels near the Golan will heed calls from exile-based leaders. Rebel fighters tend to see the opposition figures in exile as out of touch.

The abduction of the peacekeepers came a week after the announcement that another member of UNDOF went missing.