* Assad's forces moving chemical weapons - Dempsey
* U.S. to discuss more military collaboration in Jordan
* Dempsey to meet his Israeli counterpart and Netanyahu
By Phil Stewart
TEL AVIV, Aug 12 (Reuters) - The United States is gaining more insight into Syria's moderate opposition but must watch carefully to determine when occasional collaboration with Islamist radicals might turn into real alliances, the top U.S. military officer said on Monday.
The comments by General Martin Dempsey, chairman of the U.S. military's Joint Chiefs of Staff, came at the start of a trip to close U.S. allies Israel and Jordan likely to be dominated by discussions about Syria's conflict and broader regional unrest.
Syria's Western-backed rebels have been clamoring for the United States to make good on promises to provide weapons. But the Obama administration has been slow to act because of concerns that American arms could find their way to al Qaeda-linked fighters.
Dempsey, who did not discuss in any detail the issue of weapons in his remarks to a small of group of reporters in Tel Aviv, said some amount of collaboration between moderate and extremist rebels was "unsurprising" given their shared goal to oust President Bashar al-Assad.
"The real challenge for the intel community, frankly, is to understand when they're collaborating just for a particular issue at a particular time and when they may actually be allied with each other," he said.
"And to this point, I think, we're not exactly certain where that fine line of distinction might reside."
Dempsey said perhaps the single greatest point of collaboration between the United States, Israel and Jordan on Syria centered on the threat from Assad's chemical weapons as the civil war rages.
Dempsey renewed U.S. assertions that Assad's forces were sometimes moving the chemical weapons.
"We know for a fact that it is moving from time to time," Dempsey told reporters, adding the U.S. believed Assad's forces were moving the weapons to keep them secure.
Asked whether there was any recent movement, Dempsey said: "It's a frequent occurrence, and I think that's probably a reflection of the fact that the regime is concerned that if it were to place it in one place on a permanent basis, it could be vulnerable."
'WHAT ELSE?' FOR JORDAN
Dempsey will meet Israel's military chief of staff Lieutenant General Benny Gantz - someone Dempsey has seen more of than any other counterpart - and Prime Minister Benjamin Netanyahu on Tuesday, with talks expected to include Syria.
In Amman, Dempsey said he would discuss possible ways to further bolster Jordan, where around 1,000 U.S. forces are now stationed, including troops operating F-16 fighter jets and Patriot missile batteries to deter any Syrian attack.
He raised the possibility of working together on intelligence, surveillance, and reconnaissance (ISR) - which can be done with manned or unmanned aircraft.
"I'll be in discussion with my Jordanian counterpart about 'What else?'," Dempsey said.
"Is it additional ISR collaboration? Is it border control? ... There (are) some things that we do better than anyone else in the world and that we can share with willing partners if the need arises."
Dempsey said it was important to take a longer view of the Syrian conflict and its regional fallout. More than 100,000 people have died so far in the civil war and 1.7 million Syrians have been forced to flee to neighboring countries.
"This is a regional conflict that stretches from Beirut to Damascus to Baghdad and it's the unleashing of historic ethnic, religious and tribal animosities," Dempsey said.
"It will take a great deal of work, and a great deal of time, to resolve." (Editing by Eric Walsh)