The United Nations warned on Friday that half of all Syrians will need humanitarian aid by the end of 2013 and launched what it said was the largest emergency appeal in history to cope with the civil war crisis.
"Syria as a civilization is unraveling," said the U.N. High Commissioner for Refugees, Antonio Guterres, announcing the call for some $5 billion before the end of the year.
The joint statement by U.N. 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 capitalize on their own recent gains.
Clashes also 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 U.N. monitoring mission in the Golan, announced on Thursday it was withdrawing from the area because of the violence, jeopardizing 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.
"But this will happen, of course, only if the regional powers show interest, and if the U.N. secretary general asks us to do so," Putin said at a meeting with Russian officers.
Highlighting the scale of the crisis, U.N. 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 neighboring countries hosting refugees," Guterres said.
Judging by the current refugee flows, the United Nations also forecast that the Syrian refugee population would double over the coming seven months to 3.45 million from 1.6 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 that there were many bodies in the fields, including some women and children, adding it was impossible to collect the corpses.
Seeking to regain some lost momentum, rebels said they were close to capturing the Minagh air base in northern Aleppo province, close to the Turkish border.
"There has been a significant advance by the rebels today despite incredibly violent shelling by the regime forces," said Rami Abdelrahman, head of the Britain-based, pro-opposition Syrian Observatory for Human Rights.
"We still cannot say if or when the rebels can take the whole base, but they have made a major push."
One of the main problems faced by the opposition has been the fragmentation of rebel groups, with Islamist militia increasingly muscling out more moderate forces that had 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.
Hezbollah was founded to fight Israel and was widely respected around the Muslim world, especially in the Gaza Strip, a Palestinian enclave ruled by the Sunni Islamist Hamas group.
Gaza preacher Imad Al-Daya, who is not from Hamas, called Hezbollah chief Hassan Nasrallah the "quack of resistance" and urged Sunni youth to "wake up this a war of religion".
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."