One million Syrians have fled their homeland since a revolt erupted two years ago, the UN said on Wednesday, as monitors said regime warplanes struck rebel enclaves in flashpoints across the country.

"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," UN High Commissioner for Refugees Antonio Guterres warned.

"We are doing everything we can to help, but the international humanitarian response capacity is dangerously stretched. This tragedy has to be stopped," he said in a statement issued in Geneva.

Only a year ago, the UN agency had only registered 33,000 refugees. The exodus has intensified this year, the UNHCR said, with 400,000 Syrians fleeing their country since January 1, mostly to the country's neighbours.

Guterres's announcement came as the UN emergency relief coordinator, Valerie Amos, said four million people in Syria need more than $1.4 billion in aid over the next four months.

On the battlefront, Syria's northern city of Raqa came under total rebel control on Wednesday, two days into battles with troops loyal to President Bashar al-Assad's regime.

"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 for Human Rights 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 Britain-based Observatory said.

Fighter jets also bombarded Homs in the centre, on the fourth day of a major offensive aimed at crushing the insurgency in the country's third-largest city, it said.

Near Damascus, the air force bombarded several rebel enclaves in the Eastern Ghouta area, including the battered town of Douma, a rebel stronghold, 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.

On Wednesday, armed men also detained about 20 UN peacekeepers in the Golan Heights between Syria and Israel, a UN spokesman said.

The Observatory said the peacekeepers were from the Philippines, but the UN did not give the nationalities of the UN Disengagement Force (UNDOF) troops.

The watchdog distributed two amateur videos that carried statements by the rebel Yarmuk Martyrs Brigade claiming the peacekeepers' capture.

Meanwhile, on a visit to the European Parliament in Brussels, the chief of staff of Syria's rebel Free Syrian Amy said Assad's regime could be toppled "within a month" if Western nations agreed to arm 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 a "non-lethal" package of 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 said it is prepared to hand Syria's seat in the organisation to the opposition if it sets up an executive body.

It 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.

Elsewhere on the diplomatic front, 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 it opposes calls for Assad to quit before talks between the regime and opposition.

US Secretary of State John Kerry, in comments aired on Wednesday, said "a lot of countries" are training Syrian rebels as part of stepped up efforts to topple Assad.

"It's one part of it. But other nations are doing other things. There are a lot of nations working at this. And so I think President Assad needs to read the tea leaves correctly," Kerry told Fox News.

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