Even with the rebel advances, loyalists in the Syrian military appear to be holding firm.

Much of the anti-Assad fighting force includes military defectors. There are signs that military defections at the commander level are slowing though the U.S. analysts are not sure why, the second U.S. official said.

"There is still regime control over the military despite the fact they recognize the opposition force has improved," said the second U.S. official said.

Syrian rebels, government battle

The Syrian civil war started in March 2011 when a government crackdown on civilian demonstrators morphed into a fight between the regime and rebels.

The conflict has seized the attention of world powers for months because of the relentless brutality and the specter of the Syrian government mulling the use of chemical weapons.

The war has a proxy element, with Sunni countries such as Turkey, Qatar and Saudi Arabia backing the rebels and Shiite Iran backing the Alawite regime. The Alawite faith is an offshoot of Shiism.

More than 40,000 people have died in the war. The United Nations said on Friday that many Syrians will continue to be killed and maimed after the war ends because of deadly explosives placed in residential areas across the country.

The violence continued Friday, with at least 32 deaths recorded by the opposition Local Coordination Committees said.