Some of Syria's strongest Islamist rebel groups have joined forces to reject the Western-backed Syrian National Council and form a new alliance that could significantly alter the dynamics of the 2-1/2-year-old uprising against the regime of President Bashar Assad.

The Islamic Coalition, if it sticks together as a cohesive force amid the constant shifting of rebel loyalties, underlines the growing power of highly motivated, hard-line Sunni factions and the weakness of the bickering exile-run nationalist groups backed by an indecisive West.

The growing power of the Islamist forces, who now dominate the uprising, is an awkward reality for the Western powers who deal only with secular SNC.

The new alliance comprises 13 Islamist groups, including some of the toughest factions like Jabhat al Nusra, or Victory Front, the largely homegrown al-Qaida affiliate in Syria, and Liwa al-Tawheed, or Tawheed Brigade, one of the largest and most effective rebels group in the north.

Between them they can muster some 20,000 fighters. They control much of northern Syria.

Emphasizing the amorphous nature of the disparate anti-Assad opposition, three of the groups to join the new alliance are considered moderate Islamists who were affiliated with the Istanbul-based Syrian National Coalition, the umbrella of the Syrian opposition.

These are Liwa al-Tawheed, Liwa al-Islam and Suqor al-Sham. Their defection underlines not just the Islamist rebels' growing influence but the Islamization of some of the more nationalist groups that form the Free Syrian Army.

Only last week, the outgunned 11th Division of the Western-backed FSA joined up with the Jabhat al-Nusra in the northern city of Raqqa, the province where al-Qaida's presence is strongest, when faced with the Islamic State of Iraq and the Levant, the other main al-Qaida affiliate in Syria.

The 11th Division's defection illustrated how bizarre the rebels' constant shifting has become. It hooked up with one jihadist group while fighting another jihadist faction.

And to complicate matters even more, ISIL, one of the strongest rebel forces, and the smaller al-Nusra were fighting each other in the north after months of deepening hostility between the ultra-hard-line Islamic State and the smaller but often effective al-Nusra.

The formation of the new Islamic coalition followed a sharp escalation in fighting between ISIL and the FSA in the north, where the al-Qaida hard-liners have been alienating the population with their extremist behavior.

These events have been particularly heavy blows for the SNC and its Western supporters.

The new alliance is likely to complicate U.S.-led international efforts to get the Damascus regime and its enemies to the negotiating table to end a conflict that has killed more than 100,000 people and displaced up to 6 million refugees.

"This all benefits Bashar Assad enormously," Beirut analyst Michael Weiss wrote on the Now Lebanon website.

U.S. President Barack Obama's retreat from unleashing punitive military action against Damascus for allegedly using chemical weapons to kill 1,400 people in Damascus Aug. 21 was a major let-down for the rebels.

It seriously undermined their expectations of military strikes, however symbolic, against Assad's strategic centers.

"The inclusion of the core of the National Coalition's force ... effectively depletes its armed wing, the Supreme Military Council," said Charles Lister of IHS Jane's Terrorism and Insurgency Center in London.

"It's likely that the moderate Islamist coalition has ceased to exist as a single organization."

"The truth is that the West has lost Syria," observed Beirut-based commentator Ana Maria Luca, "simply because they deceived the Syrian people into waiting 2-1/2 years for international support that never came...

"Now, the Syrians' hope is gone. Eventually they turned to the only faction that can actually give them a chance to earn their freedom. They don't care what the West thinks about it."

Rebel chieftains in the war-battered northern city of Aleppo, once Syria's commercial heart and long a major battleground, have been particularly angered by Obama's decision to put military strikes on hold while focusing on U.S.-Russian diplomacy.

They had been planning an offensive to coincide with the expected U.S. operations, in which they'd hoped to make advances under cover of American airstrikes.

Now those hopes have been dashed and there's only anger at the United States.

"This was about the Americans saving their reputation, not about helping us," one military council chieftain complained. "Our reality is that nothing will change for us."