George Sabra, interim head of the National Coalition, issued the rejection after President Bashar al-Assad’s forces seized all of the Qusayr area in central Syria.
“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.
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.
He had already said on May 30 that the opposition would not attend a peace conference while Iran and the Lebanese Shiite movement Hezbollah were supporting troops on the ground.
The seizure of Eastern Bweida village, the last rebel bastion in the area, brought the entire Qusayr region near the Lebanese border back under regime control.
That came four days after the town of Qusayr, which had been in insurgent hands for a year, fell to the army and fighters from the Lebanese Shiite movement Hezbollah.
Sabra charged Hezbollah, along with majority-Shiite 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 Shiite Islam, while the rebels fighting it are mostly Sunni Muslims.
He also repeated previous statements by rebels that they have the right to defend Syria, warning the Lebanese government that it would have to take responsibility for the “implications of the invasion” by Hezbollah.
Hundreds of people who fled Qusayr had taken refuge in Eastern Bweida, 14 kilometres (nine miles) to the northeast, but Syrian state television broadcast footage of a desolate village devoid of signs of life.
Qusayr, only 10 kilometres from Lebanon, is strategic for the regime because because it lies on a route linking 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 were evacuated to Arsal, a border town in northern Lebanon, and to Baalbek in the east, security officials said.
Among them were Lebanese Sunni Muslims who had crossed into Syria.
At the same time, scores of families fleeing the area have also braved a dangerous journey to safety in Arsal, local official Ahmad al-Hojeiri said.
“We have many people arriving today. Among them were some 50 injured. Some had light wounds; some were in bad shape.
One man told him “there was an agreement to allow people to leave Eastern Bweida. When they left, they were ambushed. Most of the people in his group were killed or injured.”
Hojeiri said one civilian from Qusayr “was trying to evacuate his wife and his two children… when a shell dropped. He lost his family.”
“I’ve been seeing injured people and refugees arrive here in Arsal since the start of the Syrian revolution. But I’ve never seen anything this bad.”
More than 500,000 Syrians fleeing the conflict have sought refuge in Lebanon, which is increasingly being sucked in to the war.
Indeed, Syrian helicopters fired rockets late on Friday at an area near Arsal, whose residents back the rebels, in the second such strike in less than a week.
The latest violence comes as the United Nations said a total of $3.8 billion was needed to help refugees who have spilled across the country’s borders to escape the fighting.
The figure needed for operations inside Syria was put at another $1.4 billion.
“If the fighting doesn’t stop, we risk an explosion in the Middle East for which the international community is not prepared,” UN refugee agency head Antonio Guterres said.
More than 94,000 people have been killed and some 1.6 million Syrians fled the country since the civil war began in March 2011 after Assad cracked down on pro-democracy protests, according to Observatory figures.
The number of refugees is expected to reach at least 3.45 million by the end of this year, according to the UN appeal.
Inside the country, 6.8 million people, most of them displaced from their homes, are forecast to need aid this year.