U.S. officials said they believe al-Assad is still controlling some of his military forces and commanders. He and his top advisers are showing less ability to maintain control than they did six to eight months ago.

"There has been a strongly downward steady progression" in al-Assad's grip on power," said the senior U.S. official

The United States believes "the wall around him is slowly coming down," said the senior official of the strong inner circle around the Syrian president. "We are saying there are indicators there is weakening around Assad."

But there is no indication al-Assad is making plans to step down, the senior official and other administration officials said.

A second official said al-Assad and his commanders appear to be fully aware the opposition has made significant military gains in recent weeks and that al-Assad "is not out of it" in his understanding of the current situation.

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.