UN officials said negotiations were under way Thursday with Syrian rebels who seized 21 UN peacekeepers in the Golan Heights ceasefire zone between Syria and Israel, as rebels took total control of the key northern city of Raqa.
Syrian rebels on Wednesday abducted the peacekeepers, who are from the Philippines, diplomats said, as the frontiers of their war against President Bashar al-Assad spread further.
UN Secretary General Ban Ki-Moon called for their "immediate" release and demanded the Syrian government and rebels respect their "freedom of movement and security".
The UN said it was trying to negotiate the release of the soldiers, while a rebel spokesman said the troops would be held until Assad's forces pull back from a Golan village.
Officials in Manila urged the troops, who are part of a 300-strong Filipino peacekeeping unit, to be released immediately. Philippine armed forces spokesman Colonel Arnulfo Burgos told AFP the rebels were treating the hostages well.
"They are being treated as guests, not as hostiles," Burgos said, adding: "We have high hopes that they are going to be released soon."
Russia's UN ambassador Vitaly Churkin, who read the Security Council statement on the situation, said the rebels have made demands directed at the Syrian government, but gave no details.
The United Nations has reported a growing number of incidents in the Golan over the past year. It has sent extra armored vehicles and communications equipment to reinforce security for the mission.
The Britain-based watchdog Syrian Observatory for Human Rights distributed two amateur videos with statements by the rebel Yarmuk Martyrs Brigade claiming the capture.
A man identified as brigade spokesman Abu Kaid al-Faleh said the peacekeepers would not be freed until Syrian regime forces pull out of the area.
"If they do not withdraw, these men (UN troops) will be treated as prisoners," he said, accusing the UN Disengagement Force (UNDOF) -- which monitors a 1974 ceasefire deal between Syria and Israel -- of working with the army to try to suppress the insurgency.
UN peacekeeping chief Herve Ladsous told reporters after briefing the Security Council that negotiations are underway "and the matter is mobilizing all our teams".
Meanwhile, UN High Commissioner for Refugees Antonio Guterres warned that one million Syrians have fled their homeland since the revolt erupted two years ago.
"With a million people in flight, millions more displaced internally and thousands of people continuing to cross the border every day, Syria is spiralling towards full-scale disaster," Guterres said.
"We are doing everything we can to help, but the international humanitarian response capacity is dangerously stretched. This tragedy has to be stopped."
Medecins Sans Frontieres (MSF) called for a political agreement to allow humanitarian aid to be distributed.
MSF also stressed the growing problems of Syria's neighbours, as each day 5,000 refugees, mainly women and children, reach crowded and poorly-equipped camps across the borders.
On the ground, the northern city of Raqa came under total rebel control on Wednesday, two days into battles with troops loyal to President Bashar al-Assad.
"Raqa city is now out of the army's control, after military intelligence troops surrendered to rebels following fierce clashes that raged for two days," Syrian Observatory director Rami Abdel Rahman told AFP.
"It is the first provincial capital out of regime control."
Earlier, an air raid on Raqa killed and wounded dozens of people, the Observatory said. Warplanes also bombarded Homs in central Syria, on the fourth day of a major offensive in the country's third-largest city.
Near Damascus, the air force bombarded several rebel enclaves, said the watchdog which relies on a vast network of activists and medics on the ground.
The United Nations says at least 70,000 people have been killed since the start of the uprising, and the Observatory said at least 121 were killed on Wednesday.
Visiting the European Parliament in Brussels, the chief of staff of the rebel Free Syrian Amy said Assad's regime could be toppled quickly if Western nations armed the insurgency.
"What we have now is little, very very little," said Brigadier General Selim Idriss. "If we have the weapons we need, we can bring down the regime in a month."
Britain's Foreign Secretary William Hague said London will provide body armour and armoured vehicles to the rebels, as part of "non-lethal" aid worth $20 million.
"The fact remains that diplomacy is taking far too long and the prospect of an immediate breakthrough is slim," he told parliament after the European Union authorised the supply of non-lethal military gear and training to Assad's foes.
In Cairo, the Arab League called for the opposition National Coalition "to form an executive body to take up Syria's seat" and attend its next summit, in Doha on March 26-27.
Russia said its point man on Syria would meet his US counterpart and UN-Arab League envoy Lakhdar Brahimi in London on Thursday.
Moscow is upset with a US decision to also step up non-military support for the rebels, and opposes calls for Assad to quit before talks between the regime and opposition.