* West jittery, divided over growing strength of Syria Islamist rebels
* Western-backed opposition stutters, lacks unity, credibility
* Syrian coalition could become more conservative if relies on Gulf backing
By John Irish and Mohammed Abbas
PARIS/LONDON, Jan 25 (Reuters) - Western concern over the growing strength of jihadist rebels in Syria is mounting, hindering aid to the moderate Syrian National Coalition opposition and possibly pushing it into the arms of religiously conservative backers, diplomatic sources said.
The widely recognised coalition has failed to gain traction on the ground in Syria since being formed in November, its credibility undermined by its failure to secure arms and cash in the battle to oust Syrian President Bashar al-Assad.
Meanwhile, the coalition's lack of cohesion - it this week failed to form a transitional government - has deterred the West from boosting aid to the group, in particular the guns and ammunition coalition fighters are crying out for.
That has left the door open to Islamist groups, funded and armed by wealthy Gulf states and individuals, to become the strongest fighting factions in Syria. They command local respect for their effectiveness, but alarm some in the West.
On Monday, Western and Syrian coalition officials hope to break the deadlock at a meeting in Paris, amid coalition accusations of broken promises of aid and splits in the West on how to address the Islamist presence in the Syrian rebel ranks.
"This meeting is to ring the alarm bell. We have to assure the coalition of our support and the support of the international community," said a French diplomatic source.
"We must avoid a government in exile. The objective is to have a direct impact on the ground. Bring value to the Syrians on the ground," the source added.
Syrian coalition officials say the best way to make an impact is to provide its poorly equipped fighters with weapons.
But Western diplomats are wary of the coalition's disunity, and are mindful of the spread of weapons to Islamists in Syria and across the volatile region.
French forces are currently battling Islamists in Mali, the insurgents armed with weapons thought to have come from Libya after the Western-backed 2011 uprising against Muammar Gaddafi.
"We have also learnt from experience and we're seeing it in Mali with weapons that came from Libya to the armed groups there now. What we don't want is weapons falling into the hands of the wrong people," the French source said.
It would have been easier to arm the rebels had they agreed to form a transitional government, the source added.
Complicating matters are apparent divisions on how to handle Islamist groups such as the al Qaeda-linked Jabhat al-Nusra, one of the most coherent and disciplined anti-Assad forces in Syria.
The United States has proscribed the group as a terrorist organisation, and Britain is understood to share its concerns.
"It is a concern that the strongest groupings are Islamist fighters possibly linked to al Qaeda, and that clearly is a vital national interest for us to make sure that does not happen," said a Western diplomat on condition of anonymity.
But the Syrian coalition, mindful of alienating a group fighting for Assad's downfall, criticised the U.S. move, while France downplayed the Islamist influence.
"There are a multitude of groups and factions, political or military allegiances changing all the time ... we don't believe there can be an Islamist front," the French source said.
Meanwhile, there may be pressure on the Syrian coalition to turn to more religiously conservative backers, said mainly to be states and wealthy individuals in the Gulf, for arms and cash in the absence of aid from the West.
In the midst of negotiations in Istanbul to form a transitional government, Syrian coalition leader and moderate Sunni cleric Moaz Alkhatib flew to Sunni Muslim Qatar try to secure financial aid, opposition sources said on Sunday.
Conservative Gulf backing could make the coalition less inclusive, alienating Syrian minorities such as Alawites, Christians and Kurds and fanning ethno-sectarian violence.
Western powers are keen to avoid such an outcome.
"One of the main objectives of my diplomatic engagement and those of my Western colleagues is to keep the pressure on the national coalition to expand into the centre ground of Syrian opinion," said the Western diplomat.
The coalition has dismissed such concerns, but fears are mounting among Syria's minorities.
"A good number of Christians and Alawites don't see themselves represented neither in the regime nor the insurrection which they fear is increasingly dominated by radical Islamists," former senior Syrian military official and prominent defector Manaf Tlas told France's Le Monde newspaper. (Writing by Mohammed Abbas)