The United Nations has warned that half of all Syrians would need humanitarian aid by the end of 2013 and launched what it said was the biggest emergency appeal in history to cope with the civil war crisis.

"Syria as a civilisation is unravelling," said the UN High Commissioner for Refugees, Antonio Guterres, announcing the call for some $5 billion before the end of the year.

The joint statement by UN agencies coincided with heavy fighting on numerous fronts, as rebels attacked an air base in northern Syria while forces loyal to President Bashar al-Assad sought to capitalise on their own recent gains.

Clashes continued on the Golan Heights, close to the ceasefire line between Israel and Syria, a day after rebels briefly seized the sole crossing between the two foes.

Austria, a major contributor to a UN monitoring mission in the Golan, announced on Thursday it was withdrawing from the area because of the violence, jeopardising an operation that has helped keep the Israeli-Syrian war quiet for four decades.

Russian President Vladimir Putin, who has backed the Syrian government since the start of the unrest in March 2011, said Moscow was ready to replace the Austrian peacekeepers.

A UN spokesman said however the four-decade-old agreement on the mission in the Golan did not allow permanent U.S. Security Council members like Russia to send troops.

Highlighting the scale of the crisis, UN humanitarian agencies in Geneva said 10.25 million Syrians would need aid by the end of the year at a cost of $5 billion.

"The funds we are appealing for are a matter of survival for suffering Syrians and they are existential for the neighbouring countries hosting refugees," Guterres said.

The appeal comprises $2.9 billion for refugees, $1.4 billion for humanitarian aid and $830 million for Lebanon and Jordan, the biggest recipients of Syrian refugees.

Judging by current refugee flows, the United Nations forecast the Syrian refugee population would double over the coming seven months to 3.45 million.

Refugees are housed in often squalid camps across Lebanon, Jordan, Turkey, Iraq and Egypt. Lebanese media reported this week that the country might seek to halt the flow, worried that the Syrian war will ignite sectarian hatred at home.

The Lebanese Shi'ite group Hezbollah has poured its men into Syria to help Assad battle the mainly Sunni rebels, playing a crucial role in the capture earlier this week of Qusair - a town on a key route linking the capital Damascus to the coast.

Syrian forces moved on Friday to flush out remaining pockets of resistance around the town. Assad's army and Hezbollah fighters are expected to turn their attention in the coming days to rebel positions around the northern city of Aleppo.

Activists said there was heavy fighting in orchards surrounding Qusair as well as the town of Husseiniya. They said there were many bodies in the fields, including those of women and children, and it was impossible to collect the corpses.

The UN Security Council urged Damascus to give "immediate, safe and unhindered access ... to relevant humanitarian, including UN, actors, to reach civilians in al-Qusair, in urgent need of assistance, in particular medical assistance."

Seeking to regain lost momentum, rebels said they were close to capturing the Minagh air base in northern Aleppo province, close to the Turkish border.

But the rebel side is fragmented, with Islamist militia increasingly muscling out more moderate forces that have been courted by Western governments.

A video posted on the Internet on Friday showed Islamists executing two rebels, one named Mahmoud al-Majdami, who was the leader of an opposition brigade, and his aide, Mahmoud al-Abed.

The two men, blindfolded and kneeling on rugs, were accused of theft, blackmail and murder. "I don't know about that, please excuse me," one of the men says shortly before he is shot dead.

In a separate video, fighters discarded a flag that had become synonymous with the Syrian revolt in a rebel-held part of Aleppo and replaced it with a black Islamist flag. "They've taken down the flag of the revolution and thrown it away as if it was the Israeli flag," says the activist filming the scene.

The Syrian war, which has left at least 80,000 dead, has worsened sectarian divisions across the Middle East, and a number of prominent Sunni mullahs denounced on Friday the participation of Shi'ite Hezbollah, which is allied to Iran.

Lebanon, which was devastated by its own civil war from 1975-90, feels especially threatened by the conflict, with small-scale clashes linked to the Syrian tensions reported in numerous parts of the country in recent weeks.

Looking to restore calm, the army command issued a statement on Friday saying it was doing all it could "to prevent attempts at dragging Lebanon into the Syrian conflict".

It added: "The military leadership ... calls on citizens to be aware of what is happening around them in terms of plots to push Lebanon backwards and drag it into a ridiculous war."

Alarm at the spread of the Syrian turmoil has added urgency to peace efforts. The United States and Russia hope to bring the warring sides together for talks in Geneva, possibly in July.

One UN agency, the World Food Programme (WFP), has delivered 500 million meals in Syria so far this year.

"We have reached a stage in Syria where some of the people, if they don't get food from the World Food Programme, they simply do not eat," the WFP's Syria Regional Emergency Coordinator Muhannad Hadi said.

The biggest donors to the UN aid effort are the European Commission, which promised another 400 million euros on Thursday, followed by Kuwait, the US, Britain and Japan. Russia and China have so far contributed $10 million and $1 million to the UNHCR, Guterres said.

Russia's Ambassador in Geneva Alexey Borodavkin said on Friday however he had just received instructions from Moscow to announce a contribution of tens of millions of dollars.

He said Russia also supplied help worth tens of millions of dollars directly to Syria and millions more to the International Committee of the Red Cross. Other nations that had pledged money had not honoured their promises, he said.-Reuters