* U.N. mission threatened by Syria violence
* Croatia pulling troops out after weapons report
* Rebels say UNDOF legal adviser held for ransom
* Diplomats worry UNDOF's future may be in doubt
U.N. peacekeepers monitoring the ceasefire line between Syria and the Israeli-occupied Golan Heights have halted patrols after rebels detained 21 observers for three days last week, diplomats said on Thursday.
The seizure of the unarmed observers, all Filipinos, highlighted the vulnerability of the 1,000-member U.N. Disengagement Observer Force to the growing violence in Syria. Diplomats in New York expressed concern that the future of UNDOF, whose mission began in 1974, may now be in doubt.
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 al-Assad and his father before him on the Golan front.
A senior Western diplomat in New York said on condition of anonymity that the U.N. peacekeeping department "decided to restrict the movement of UNDOF. So they're no longer doing patrols, they've closed down some of the observation posts, and they're retreating to a safer part of the area."
A U.N. official in Damascus declined to comment, but two Israeli officials confirmed that UNDOF had curtailed 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 in Syria. 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.
Until recently, four countries provided the just over 1,000 military observers in UNDOF: Austria, Croatia, India and Philippines. A senior Western diplomat said the Philippines were also considering pulling out.
That would leave India and Austria, which diplomats say makes up the bulk of the remaining UNDOF personnel.
WILL UNDOF PULL OUT?
"The Austrians have now written in expressing concern about the safety of their people," the Western diplomat said. He added that there were now concerns that UNDOF, whose observers only carry sidearms and cannot defend themselves against automatic weapons, might be unravelling.
"It's very important symbolically that UNDOF is there because the last thing we want is for the Syrian crisis to spill over into Israel in a dramatic way," he said.
Another Western diplomat said, "One more incident and they will pull out."
UNDOF has the task of monitoring "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.
U.N. 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.