A decision by key Syrian rebel groups to break with the Western-backed National Coalition further splinters the opposition and poses a key challenge for its international supporters, analysts say.

A new alliance sees members of the military command of the West's ally General Salim Idriss join forces with the Al-Qaeda-affiliated Al-Nusra Front.

It effectively guts the Syrian Military Council (SMC) that Idriss heads, and raises questions about how much influence the West and other rebel backers will now have on the ground, experts say.

And both the language of the announcement and the participation of Al-Nusra will raise fears in the West about increasing radicalisation in the armed opposition.

The announcement by 13 rebel factions, including the powerful Ahrar al-Sham, Liwa al-Tawhid and Liwa al-Islam groups, came on Tuesday night.

In a statement, they said the key opposition grouping known as the National Coalition to which Idriss belongs "does not represent us, nor do we recognise it."

The group called "on all military and civilian groups to unite in a clear Islamic context that... is based on sharia (Islamic) law, making it the sole source of legislation."

The announcement strips the mainstream rebel force of some of its most important players, according to Charles Lister, an analyst at IHS Jane's Terrorism and Insurgency Centre.

"It's extremely damaging," he told AFP.

The 13 groups "represent a very significant portion of the armed opposition and the groups that have had the most strategically valuable impact."

"The impact this will have on the ability of the SMC to represent itself as the core of the opposition will be huge."

Aron Lund, a Syria expert, agreed, in an analysis posted on the Syria Comment blog.

"It represents the rebellion of a large part of the 'mainstream Free Syrian Army' against its purported leadership, and openly aligns these factions with more hardline forces," he wrote.

Thomas Pierret, a lecturer at the University of Edinburgh, pointed out that last year some of the factions had refused to join Al-Nusra in a similar denunciation of the Coalition.

He said a US-Russian deal to strip Syria of its chemical weapons, which halted US plans for military action against the regime, had changed the calculus for some.

"After the chemical weapons crisis, insurgents have lost any possible hope in the benefits of an alliance with the West," he said.

While the Islamist bent of the alliance is clear, many of Syria's rebel groups use Islamist language, and Lister said he did not expect to see all the factions fall in line with Al-Nusra.

In fact, analysts pointed to a notable jihadist absence from the new grouping -- that of the Al-Qaeda-affiliated Islamic State of Iraq and the Levant (ISIL).

The group, which began as Al-Qaeda's Iraqi arm and expanded into Syria earlier this year, has clashed with other rebel groups in recent months.

Lund said it was too early to see the new grouping as intended to confront ISIL, noting that nothing in its statement would be objectionable to the Al-Qaeda affiliate.

"It might in fact suit them pretty well, since it weakens the hand of the Western-backed camp and adds weight to Islamist demands," he wrote, adding that ISIL could even join the alliance going forward.

But Lister said ISIL's exclusion was "not accidental."

"The fact that there's nothing in the statement that ISIL would disagree with and yet they're not included is significant in and of itself."

Pierret added that the group's Islamist language "may be a means for the signatories to bolter their Islamic credentials in anticipation of a possible future step against ISIL."

'More and more difficult to provide arms'

Even without ISIL, the alignment of formerly Western-backed brigades with Al-Nusra will pose problems for the West, much of which lists the group as a "terrorist organisation."

Washington has been talking about ramping up military aid to the rebels through Idriss, but the new alliance could make that impossible.

"They've got no way forward, they've put themself in a box. It becomes more and more difficult to provide arms," said Yezid Sayigh, a senior associate at the Carnegie Middle East Centre.

The decision by key rebel factions to reject the National Coalition could further diminish the chances of a negotiated solution to the conflict, with the Coalition no longer able to claim it represents the opposition.

"The National Coalition wasn't exactly going anywhere anyway... (but) its lack of meaningful control or influence is just being highlighted all the more," Sayigh said.

"Bottom line is that the issue of representation is going to be massive one."

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