* Muslim militants gain if world does not pitch in - Fabius
* Syrian opposition says does not want empty promises of aid
* Say needs $500 million to set up transitional government (Adds comments from diplomatic source)
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, tackled 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.
The insurgents now control parts of the north and east of Syria, and have made inroads in major cities. But the air power and far superior weaponry of Assad, whose family has been in power for 43 years, have limited rebel advances.
Fabius sidestepped the question of arming the rebels, underlining the Western wariness about spreading weapons to Islamists across the volatile region, where long-standing rulers in Tunisia, Egypt and Libya have been toppled in popular uprisings over the past two years.
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.
The European Union is set to review its arms embargo on Syria at the end of February.
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.
"From the beginning we said we should be based in Syria, but so far we haven't received any money to run a government," Seif said.
He 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.
A French diplomatic source present at the meeting said the coalition had received concrete promises from several countries that should keep it running for several months. He declined to say how much or which countries.
He acknowledged, however, that the meeting would not provide a magic solution.
"The risk (of doing nothing) is the loss of credibility for the coalition within the country and that is what we are trying to avoid," the source said. "If we don't have an alternative for the people, they will look where there is a bigger response."
But the coalition's disunity - it failed last week to form a transitional government - has deterred the West from boosting assistance, especially sophisticated arms and ammunition that the 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?" (Editing by Louise Ireland)