DAMASCUS: Syrian rebels held hostage 21 UN peacekeepers patrolling the sensitive armistice line with Israel for a second day on Thursday, defying a chorus of international condemnation and calls for their release.

Diplomats scrambled to secure the release of the 21 Filipinos, as concern mounted that their seizure might prompt more governments to withdraw their contingents from the already depleted UN mission.

Israeli officials warned that any further reduction in the strength of the UN Disengagement Observer Force (UNDOF) risked creating a security vacuum in the no-man's land between the two sides on the strategic Golan Heights.

The Philippine soldiers were detained at a rebel post on Wednesday just one and a half kilometres (a mile) to the Syrian side of the armistice line at its southern end towards the Yarmuk River on the border with Jordan.

The rebels, calling themselves the Yarmuk Martyrs Brigade, demanded in video statements that Damascus withdraw its troops from Jamla and neighbouring villages in the area.

Manila strongly condemned the seizure of its troops and demanded they be released immediately, a call echoed by UN chief Ban Ki-moon.

Britain-based watchdog the Syrian Observatory for Human Rights said it had spoken to the rebels who had given undertakings that the hostages would not be harmed.

"They say they will treat them well, as guests, and set them free as soon as the army withdraws from Jamla and surrounding villages," Observatory director Rami Abdel Rahman told AFP.

Philippine President Benigno Aquino said he had received similar assurances. "I understand they are being treated well... so far, nobody has been saying that they are in danger," Aquino told reporters.

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 since the 1974 armistice that followed the previous year's Middle East war.

At the end of February, UNDOF 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Canada and Japan had already withdrawn their small contingents and Croatia announced last week it was pulling out its 100 troops.

The Philippine president 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 to secure the peacekeepers' release "and the matter is mobilising all our teams".

In video statements distributed by the Observatory, 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 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.

Nationwide violence claimed 179 lives on Wednesday -- 73 rebels, 57 civilians and 49 soldiers, it added.

The United Nations says that more than 70,000 people have been killed and more than one million fled the country since the revolt against President Bashar al-Assad's rule erupted in March 2011.

Medical charity Doctors Without Borders warned on Thursday that those who remained in the country face a "humanitarian catastrophe".

"Syria's previously well-functioning health system has collapsed. Food shortages are commonplace, and water and electricity supply are severely disrupted," it said.