Syria's opposition has reiterated its decision to boycott planned peace talks in Geneva, as rebel fighters reeled from losing a strategic city to forces of President Bashar al-Assad.
George Sabra, the interim head of the Syrian National Coalition (SNC), however, called for urgent military assistance to help rebel forces battle the Syrian army.
His statement came days after regime forces seized the key border city of Qusayr and other adjoining areas.
"What is happening in Syria today completely closes the doors on any discussions about international conferences and political initiatives," Sabra told a press conference in Istanbul on Saturday.
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He was referring to an initiative headed by Washington and Moscow to bring the regime and opposition to peace talks in Geneva.
"The war declared by the regime and its allies in the region has reached a level we cannot ignore," Sabra said.
Sabra had already said on May 30 that the opposition would not attend a peace conference while Iran and the Lebanese Shia movement Hezbollah were supporting Assad's troops on the ground.
Assad's forces retook Qusayr, which had been in rebel hands for a year, four days ago, and followed that up by seizing Eastern Bweida village. The capture of the last rebel bastion in the area brought the entire Qusayr region back under regime control.
Sabra charged Hezbollah, along with majority-Shia Iraq and Iran, of pushing towards a "sectarian conflict".
But he said the opposition would refuse to be dragged in, saying that this would change "our lives in the region into hell".
Assad's regime is dominated by members of the Alawite community, an offshoot of Shia Islam, while the rebels fighting it are mostly Sunni Muslims.
Sabra also repeated previous statements by rebels that they had the right to defend Syria, warning the Lebanese government that it would have to take responsibility for the "implications of the invasion" by Hezbollah.
Qusayr, less than 10 miles from Lebanon, is important for the regime as it links Damascus to the coast. For the rebels, it was an important conduit for men and supplies coming from Lebanon.
Dozens of wounded Syrians and Lebanese from the rebel side have been evacuated to Arsal, a border town in northern Lebanon, and to Baalbek in the east.
The conflict in Syria has killed more than 94,000 people. Some 1.6 million Syrians have fled the country since the uprising against Assad began in March 2011.