With talks aimed at bringing about an end to the civil war planned for November in Geneva, the opposition feels more abandoned than ever by its friends in the West.
National Coalition chief Ahmad Jarba officially welcomed the UN resolution to eradicate Syria's chemical weapons, but other members of the group expressed bitterness at the outcome.
"The (UN) Security Council resolution is a big disappointment for us," said Coalition member and veteran dissident Samir Nashar.
"It serves the interests of most regional and international powers, including the Syrian regime... but by no stretch of the imagination does it serve the Syrian people or the Syrian revolution."
The UN Security Council on Friday passed resolution 2118, after gruelling US-Russian negotiations, ordering Syria's chemical arsenal destroyed, but the resolution stops short of promising sanctions or the use of force against President Bashar al-Assad's regime.
And the resolution says nothing about the regime's use of other weapons -- from fighter jets to scud missiles -- that have also exacted a heavy toll on the rebels and civilians.
"The opposition is the big loser," France-based Middle East expert Agnes Levallois said.
"With this issue of chemical weapons, Bashar al-Assad has regained his place as the Syrian partner for talks with the international community."
Even if the Geneva talks go forward as planned, the Coalition's decisions "will neither be accepted nor recognised by the opposition inside Syria... It's terrible for the opposition," Levallois added.
Peter Harling, a Syria expert with the Brussels-based International Crisis Group, agrees that peace talks could leave the Coalition even further marginalised.
"The coalition is weak and will be further weakened by the Geneva process, if it is designed by the United States and Russia as a way of dressing up politically their rather bare agreement on chemical weapons, whose only purpose was to prevent a war neither wanted," Harling told AFP.
The UN resolution came only after the United States threatened to strike Syria over its alleged use of chemical weapons in an August 21 attack said to have killed hundreds of civilians.
Coalition 'does not represent us'
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In addition to its increasing marginalisation internationally, the Coalition also faces a major challenge from inside Syria, after some of the rebellion's most powerful factions said this week the foreign-based group no longer represents them.
The 13 factions include members of the main rebel Free Syrian Army -- which has long demanded more arms and aid from the West -- as well as more radical Islamists.
Among the signatories are Liwa al-Tawhid, the main rebel force in the northern province of Aleppo, Liwa al-Islam, which is mainly active around Damascus, and the jihadist Al-Nusra Front.
The move came amid a growing wave of accusations that the Coalition is disconnected from the suffering on the ground.
"The Coalition has drifted far away from the Syrian people, and from their reality and ambitions," said Islam Allush, spokesman for Liwa al-Islam.
Amateur video shot at an opposition meeting in Jordan and posted online showed an enraged rebel commander accusing foreign-based dissidents of abandoning Syrians.
"Who among you came to help us? Who came to bring us a glass of water?" commander Yasser Abbud, from the southern Syrian town of Daraa, tells them.
"Who among you foreign-based opponents has ever contacted us... and asked us what we need? Nobody! Nobody!" he cries.
Rebels fighting Assad's troops have long suffered from shortages of arms and ammunition, while people living in many opposition-held areas besieged by loyalist forces lack of food and other basic supplies.
Levallois said that while the foreign-based groups bear the responsibility of going to proposed talks, there is a "growing divorce between Syrians inside the country and Syrians outside."
This division "removes all the foreign-based opposition's capacity to manoeuvre at the (proposed) talks," she said.
"Whatever decisions the Coalition takes... will neither be accepted nor recognised by the opposition inside."
Louay Muqdad, the Free Syrian Army's political and media coordinator, told AFP he understands the rebels' anger at the Coalition and the international community.
"Since the very start, we have warned that abandoning the Syrian people to suffer at the hands of Bashar al-Assad... while the international community turns its back... will lead the Syrian people to despair," Muqdad said.
"Had the international community fulfilled its duties, we would not have arrived at this point."