Syrian rebels kept 21 UN peacekeepers hostage on the Golan Heights on Thursday despite world condemnation of the spillover of Syria's conflict onto the sensitive armistice line with Israel.
Diplomats scrambled to secure the release of the 21 Filipinos serving with the four-decade-old UN peace mission on the strategic plateau, as concern mounted that their seizure might prompt more governments to withdraw their contingents.
Israel, which has repeatedly warned that it will not tolerate any spillover onto the Golan, said it feared the hostage-taking might lead to the already depleted force being wound up altogether.
For months, Israel has retaliated against stray fire onto its side of the armistice line, whether from Damascus troops or from the rebels, who are also hostile to its occupation of the Syrian territory.
The Philippine soldiers were detained at a rebel post on the Syrian side on Wednesday by gunmen who said the troops would be held until regime forces pulled back from a Golan village.
Manila strongly condemned their seizure and demanded they be released immediately, a call echoed by UN chief Ban Ki-moon.
Philippine Foreign Secretary Albert del Rosario said the peacekeepers' detention was a gross violation of international law as they were operating under the UN flag.
The captive troops are part of a 300-strong Philippine contingent to the UN force that has been monitoring the separation of Israeli and Syrian troops on the Golan since a 1974 armistice that followed the previous year's Middle East war.
At the end of February, the UN Disengagement Force comprised some 1,000 peacekeepers but a growing number of incidents over the past year has made it increasingly difficult for the United Nations to keep the mission up to strength.
Canada and Japan had already withdrawn their small contingents in recent months and Croatia announced last week it was pulling out its 100 troops.
Philippine President Benigno Aquino said no decision had yet been made on the future of Manila's contingent but its withdrawal would leave just Austrian and Indian troops.
UN peacekeeping chief Herve Ladsous told reporters after briefing the Security Council that negotiations were underway with the rebels to secure the peacekeepers' release "and the matter is mobilising all our teams".
In amateur video footage distributed by Britain-based Syrian Observatory for Human Rights, a rebel spokesman said the peacekeepers would not be freed until Syrian regime forces pull out of the area.
"If they do not withdraw, these men will be treated as prisoners," spokesman Abu Kaid al-Faleh said, accusing the UN force of working with the Syrian army against the rebels.
Israel, which captured much of the Golan in the 1967 Middle East war and annexed it in 1981 in a move never recognised by the international community, said it feared any depletion of the UN force would pose a serious threat.
"This kidnapping is likely to convince countries who participate in this force to bring their troops home, which would undoubtedly create a dangerous vacuum in no-man's land on the Golan," an Israeli official said.
The top-selling Yediot Aharonot daily said Israeli officials feared that "Al-Qaeda members will take control of the buffer zone."
The rebels' seizure of the peacekeepers came as they celebrated their capture on Wednesday of military intelligence headquarters in the northeastern city of Raqa, which gave it full control of the strategic provincial capital.
Syrian fighter jets pounded the city in retaliation on Thursday, the Observatory said, while raising concern for the fate of loyalists who surrendered.
The rebels are believed to be holding the provincial governor, whom they captured during the battle, inside the building, which has now become their headquarters, the watchdog said.
"Hundreds" of pro-regime militiamen also surrendered to the rebels, it added.
Human rights groups have accused rebels and regime forces alike of carrying out atrocities against prisoners, including rebel fighters from the jihadist Al-Nusra Front, which was at the forefront of the Raqa battle.
The city's capture marked the rebels' biggest victory on the ground since the revolt against President Bashar al-Assad's rule erupted in March 2011.
They now control large swathes of both northeastern and northwestern Syria as well as a significant part of second city Aleppo and enclaves in embattled third city Homs.