BEIRUT (AP) — Clashes between Syrian troops and rebels flared close to Israeli-controlled territory Thursday, a day after rebels detained 21 UN 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 UN troops began patrolling an Israeli-Syrian armistice line in the Golan Heights nearly 40 years ago that UN 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 Six Day War, and a UN monitoring force, UNDOF, was sent in 1974, a year after the Yom Kippur 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 (0.6 mile) from Israeli-controlled territory.

On Wednesday, rebels detained 21 peacekeepers, all from the Philippines, near Jamlah. They were expected to be released Thursday, a Free Syrian Army official told the BBC in Arabic.

In an amateur video posted online Wednesday, 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 Times of Israel contributed to this report.