Washington (CNN) Syrian President Bashar al-Assad's momentum in fighting off rebel groups has decreased, but should his regime crumble, Syria could face greater instability and control by extremist groups, U.S. security officials are warning.

They attribute the gains of rebel fighters in part to their acquiring more battleground experience as well as the ability to use advanced tactics. There are also signs that moderate forces have become more coherent in their organization, making Assad more vulnerable.

But the officials pointed out that the U.S. will still have trouble identifying moderate partners while extremist groups such as al Qaeda affiliate al Nusra and ISIS, also known as ISIL, try to take advantage of any cracks in the government's control.

The Syrian regime's "momentum has been slowed," according to Joint Chiefs of Staff Chairman Gen. Martin Dempsey. "I do believe the situation is trending less favorably for the regime."

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