* No less than 1,200 opposition groups in Syria -official
* Islamist al-Nusra Front getting stronger
* Rebels frustrated at lack of Western arms
Radical Islamist rebels will gain sway over the many disparate factions opposing Syrian President Bashar al-Assad unless they are checked, and the country's civil war could last years, a top Pentagon intelligence official said on Saturday.
David Shedd, the deputy director of the U.S. Defense Intelligence Agency, did not advocate any form of intervention by the United States or its allies, saying that was up to policymakers.
But his bleak assessment of the dangers posed by the Islamist al-Nusra Front and al Qaeda's Iraq-based wing, as well as the prospects for a prolonged conflict, could bolster advocates of greater involvement by the United States and its allies.
Addressing the Aspen Security Forum in Colorado, Shedd said he counted at least 1,200 groups in the opposition. He said many of the groups were preoccupied with strictly local grievances, like a lack of potable water in their villages.
"Left unchecked, I'm very concerned that the most radical elements will take over larger segments" of the opposition groups, Shedd said, strongly hinting at the need for some kind of outside intervention.
He said the conflict could drag on anywhere "from many, many months to multiple years," and that a prolonged stalemate could leave open parts of Syria to potential control by radical fighters.
"They will not go home when it's over," Shedd said, envisioning one scenario where Assad retreats to an enclave and other parts of the country are up for grabs. "They will fight for that space, and they're there for the long haul."
Shedd added he and the DIA never thought Assad's regime would fall quickly - comments that appeared to stand in contrast to predictions by U.S. officials a year ago that Assad's days were numbered.
"DIA's position was that (Assad's fall) was no earlier than the start of this year. And it's obviously not happened," he said.
ARMING THE REBELS
U.S. plans to ship weapons to some rebels have been caught in a Washington impasse, after some members of Congress feared they would end up in the hands of Islamist militants.
Asked whether he thought more secular opposition fighters should be strengthened, or whether more radical rebel groups need somehow to be confronted, Shedd said: "I think it's too simple to say it's one or the other."
"Because it's the reality that left unchecked they will become bigger," he said, cautioning that the al-Nusra Front was gaining in strength and was "a case of serious concern."
Rivalries have been growing between the Free Syrian Army(FSA) and Islamists, whose smaller but more effective forces control most of the rebel-held parts of northern Syria more than two years after pro-democracy protests became an uprising. The conflict has killed more than 100,000 people.
The two sides previously fought together from time to time, but the Western and Arab-backed FSA, desperate for greater firepower, has tried to distance itself from the Islamists to allay U.S. fears any arms it might supply could reach al Qaeda.
Shedd's comments came as FSA rebels vent frustration at what they see as the slow pace of Western support. Britain, for example, has abandoned plans to arm rebels.
Shedd acknowledged identifying "good" versus "bad" rebels was very difficult.
"But I think (it is) a challenge that is well worth pursuing," he said.
Asked how the United States could avoid getting sucked into the conflict, Shedd said: "I believe relying on allies in the region is our best solution."
"We know that a number of the Gulf states have great concerns with the Bashar al-Assad regime. And I think that there are a number, and a sizeable number, of allies that would be prepared to work even more closely with us," he said.