One million Syrians have fled their homeland since a revolt erupted two years ago, the UN said on Wednesday, as fighting between rebels and loyalists raged across central and northern battlefields.
"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 said in a statement issued in Geneva.
"We are doing everything we can to help, but the international humanitarian response capacity is dangerously stretched. This tragedy has to be stopped."
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.
The refugees have fled primarily to Lebanon, Jordan, Turkey, Iraq and Egypt, but increasingly they are trying to reach North Africa and Europe, the UNHCR said.
The UN's announcement comes nearly two years after the eruption of what started as a peaceful uprising against President Bashar al-Assad but which escalated into civil war when the army launched a brutal crackdown on dissent.
Guterres's announcement came as UN emergency relief coordinator, Valerie Amos, said that in Syria four million people are in need of assistance and more than $1.4 billion in aid is required to assist them for the next four months.
On the battlefront, the army used warplanes and helicopters Wednesday to press a fierce four-day assault on rebel zones in the centre of Homs, eight months into a suffocating army siege of parts of the central city, said the Syrian Observatory for Human Rights.
"The army used helicopters to strafe the Old City district of Homs... and warplanes and rocket fire to strike the district of Khaldiyeh," the Observatory said.
In Syria's northeast, fighter jets struck the city of Raqa, two days after rebels overran most of the strategic provincial capital in their biggest victory since the start of the revolt, the Observatory said.
The new fighting came after at least 159 people were killed in violence nationwide on Tuesday, among them 70 rebels, 47 civilians and 42 soldiers, according to the Observatory's figures.
The United Nations says in all at least 70,000 people have been killed since the start of the uprising.
On the diplomatic front, the foreign ministry in Moscow said Russia's pointman on Syria would meet his US counterpart and UN-Arab League envoy Lakhdar Brahimi in London on Thursday to discuss the conflict.
"There is an understanding in principle for Mr (Mikhail) Bogdanov to meet in London ... with US Deputy Secretary of State William Burns" and Brahimi, foreign ministry spokesman Alexander Lukashevich said.
Russia is upset with Washington's decision to step up its non-military support for the armed rebels, and continues to resist calls for Assad to step down before the launch of any dialogue between the regime and opposition.
US Secretary of State John Kerry in comments aired on Wednesday that "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 during a visit to Qatar.
"There are a lot of countries doing training," Kerry said, without naming them.
Lebanon's foreign minister, meanwhile, called on his fellow Arab counterparts gathered in Cairo to allow the Syrian government to retake its seat at the Arab League, from which it was suspended in 2011.
Syria itself paid tribute to late Venezuelan leader Hugo Chavez on Wednesday, with state media saying he had taken an "honourable" position on the country's bloody conflict.