* Muslim militants gain if world doesn't pitch in -Fabius
* Syrian opposition: don't want empty promises of aid
* Say needs $500 million to set up transitional gov't
By John Irish
PARIS, Jan 28 (Reuters) - France's foreign minister said on Monday Syria risks falling into the hands of Islamist militant groups if supporters of the Syrian opposition do not do more to help it in a 22-month-old revolt against President Bashar al-Assad.
Addressing the opening of a conference in Paris with senior members of the Syrian National Coalition, Laurent Fabius said the meeting must focus on making the opposition politically and militarily cohesive to encourage international assistance.
"Facing the collapse of a state and society, it is Islamist groups that risk gaining ground if we do not act as we should," he said. "We cannot let a revolution that started as a peaceful and democratic protest degenerate into a conflict of militias."
Western concern over the growing strength of jihadist militants fighting autonomously in the disorganised ranks of anti-Assad rebel forces is rising. This has hindered international aid to the moderate Syrian National Coalition opposition and may push it more into the arms of conservative Muslim backers, diplomatic sources say.
The meeting, which brought together Western and Arab nations and the three vice-presidents of the coalition, aims to tackle the lack of cohesion that has led to broken promises of aid.
Coalition vice-president Riad Seif said "time is not on our side" and that the opposition no longer wanted pledges of support that would not be followed through on.
"We need an interim or transitional government to provide assistance to millions of Syrians in liberated zones and to help bring the collapse of the (Assad) regime," he said.
"From the beginning we said we should be based in Syria, but so far we haven't received any money to run a government."
HALF A BILLION DOLLARS
Since its formation in November, the coalition has failed to gain traction on the ground in Syria and its credibility has been undermined by its inability to secure arms and cash.
Seif said the coalition lacked the financial or military means to set up within Syria and support civilians on the ground. "We are looking with our friends at how we can protect the liberated zones with defensive weapons and we are discussing how to get billions of dollars to create a budget," he said.
"But if we don't have this budget there is no point having a government. It makes no sense."
George Sabra, another coalition vice-president, said the coalition needed at least $500 million to launch a government.
But its disunity - it failed last week to form a transitional government - has deterred the West from boosting assistance, especially sophisticated arms and ammunition insurgents are crying out for.
"We also need weapons. We needed them from the first minute," Sabra said. "At the last meeting of Friends of Syria, they recognised our rights to defend ourselves. (But) what does that mean if we cannot provide help to victims?"
The insurgents have seized territory in the north and east of Syria, including several border crossings, and made some inroads into Assad's dominance in major cities. But Assad's air power and far superior weaponry have limited rebel advances.
France said last week there was no sign Assad was about to be overthrown, reversing previous statements that he could not hold out long, while Jordan's King Abdullah said the authoritarian Syrian leader would consolidate his grip for now.
Fabius said the Paris meeting had three objectives: to address the needs of the vulnerable Syrian population, pursue internal structuring, bring opposition fighting units of the Free Syrian Army under its political authority and prepare the post-Assad transition.
However, he sidestepped the question of arming the rebels, underlining the wariness of Western countries about spreading weapons to Islamists in Syria and across the volatile region.
The European Union is set to review its arms embargo on Syria at the end of February. (Reporting By John Irish; Editing by Vicky Buffery and Mark Heinrich)