The UN pressed on with negotiations on Thursday to secure the release of 21 peacekeepers abducted in Syria as regime warplanes pounded the northern city of Raqa after it fell into rebel hands.

A video posted on the Internet showed footage of six members of the group of Filipino peacekeepers, who were patrolling the sensitive armistice line with Israel when they were seized on Wednesday by Syrian rebels.

One of them said they were safe and being cared for, and the UN confirmed they had not been harmed.

"The mission has been in touch with the peacekeepers by telephone and confirmed they have not been harmed," UN spokesman Martin Nesirky said in New York.

A UN official said the UN Disengagement Observer Force mission was "negotiating with the armed group and the Syrian authorities" to obtain a release.

Concern has been mounting 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 UNDOF risked creating a security vacuum in the no-man's land between the two sides on the strategic Golan Heights, which it seized in the 1967 Six-Day War.

The peacekeepers were detained at a rebel post just one and a half kilometres (a mile) on 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.

"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.

In the video footage posted on Thursday by the Syrian Observatory for Human Rights, one of the six uniformed UNDOF members said they had been travelling to Jamla when bombing and artillery fire erupted.

He said local people helped them to safety and distributed the peacekeepers to different places "to keep us safe," where they were given food and water.

SITE Intelligence Service later published a statement purportedly from the rebels saying the troops had been "secured from the barbaric bombings by Assad's criminal gangs," adding "they are in our protection until we take them to secure areas."

It called on the UN to send a "secure committee in order to hand them over to them, fearing bombing by al-Assad's gangs," saying the soliders "are now safe and welcomed and honored in the hospitality of the battalion command, until we send them safely to their headquarters."

SITE also posted a showing three of the soldiers in blue UN vest and three other clips, each showing six men.

In one of them a peacekeeper said they were be treated well by what he said was the rebel Free Syrian Army, adding however that the men hoped to see their families soon.

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

Philippine President Benigno Aquino said he had received assurances the peacekeepers would not be harmed. "I understand they are being treated well... so far, nobody has been saying that they are in danger," Aquino said.

The peacekeepers are part of a 300-strong Philippine contingent that has been monitoring the separation of Israeli and Syrian troops since the 1974 armistice that followed 1973 Arab-Israeli war.

Israel, which annexed the Golan Heights 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.

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 have already withdrawn their small contingents and Croatia said last week it was pulling out its 100 troops. If Manila pulls out it would leave just Austrian and Indian troops.

Meanwhile, violence across Syria on Thursday killed 126 people, the Britain-based Observatory said.

It also reported that fighter jets pounded the northeastern city of Raqa in a bid to recapture it.

The UN says more than 70,000 people have been killed in Syria over nearly two years of conflict, millions have been displaced and more than one million fled the country.

burs/hkb/al