BEIRUT - UN peacekeepers monitoring the ceasefire line between Syria and the Israeli-occupied Golan Heights have scaled back patrols after rebels detained 21 Filipino observers for three days last week, diplomats said on Thursday.The seizure of the unarmed observers highlighted the vulnerability of the 1,000-strong UN Disengagement Observer Force (UNDOF), whose mission began in 1974, to the growing violence in Syria.It also heightened concern in Israel that Islamist rebels, separated from Israeli troops only by a toothless U.N. force, may be emboldened to end years of quiet maintained by President Bashar Assad and his father before him on the Golan front."They have reduced their patrols for now, halted patrols in areas like the place where the Filipinos were taken hostage," one diplomat in the region said.A UN official in Damascus declined to comment, but two Israeli officials confirmed that UNDOF had reduced operations.The capture of the 21 peacekeepers was the latest challenge for the United Nations force, comprised of troops from the Philippines, India, Croatia and Austria.Japan said it was withdrawing soldiers from UNDOF three months ago in response to the violence in Syria. Croatia said last month it would also pull out its troops as a precaution after reports, which it denied, that Croatian arms had been shipped to Syrian rebels.Two weeks ago the United Nations said an UNDOF staff member had gone missing. It did not identify him but one rebel source identified him as a Canadian legal adviser and said he had been captured by another rebel force and held for ransom.The diplomat said the new restrictions on UNDOF affected mainly the southern part of its "area of separation", between Syrian and Israeli forces, a narrow strip of land running 45 miles (70 km) from Mount Hermon on the Lebanese border to the Yarmouk River frontier with Jordan."But it does affect all areas where there are potential security issues," she said, adding that the whole UNDOF operation may need to be "reframed and reworked".UN Secretary-General Ban Ki-moon said in a December report to the Security Council that fighting between Syrian armed forces and rebels inside the area of separation has "the potential to ignite a larger conflict between Israel and the Syrian Arab Republic, with grave consequences".Israel warned 10 days ago that it could not be expected to stand idle as Syria's civil war, in which 70,000 people have been killed, spilled over into the Golan Heights.The 21 Filipino peacekeepers were released on Saturday by Syrian rebels who had seized them and held them for three days in the southern village of Jamla.The rebels from the Yarmouk Martyrs Brigade initially accused the peacekeepers of collaborating with Assad's forces during heavy fighting last week and of failing to carry out their mandate to keep heavy arms away from the frontier region.At first they demanded the Syrian army cease shelling in the area and pull back from Jamla village as a condition for releasing the peacekeepers, but later described them as guests and escorted them to freedom in Jordan.