* Momentum is with opposition and Syrian people, U.S. says
* Long battle expected between pro-Assad, opposition forces
* But U.S. officials say defections are making a dent
WASHINGTON, Aug 6 (Reuters) - The White House said on Monday the defection of Syria's prime minister showed that President Bashar al-Assad's government was "crumbling from within" and repeated the U.S. call for him to step aside and end the violence gripping the country.
"This is a sign that Assad's grip on power is loosening. If he cannot maintain cohesion within his own inner circle, it reflects on his inability to maintain any following among the Syrian people that isn't brought about at the point of a gun," White House spokesman Jay Carney told a news briefing.
"The momentum is with the opposition and with the Syrian people. It's clear that these defections are reaching the highest levels of the Syrian government and Assad cannot restore his control over the country because the Syrian people will not allow it," he said.
The latest defection "only reinforces that the Assad regime is crumbling from within and that the Syrian people believe that Assad's days are numbered," Carney said.
In one of the highest-profile desertions from Damascus, Syrian Prime Minister Riyad Hijab on Monday fled the country. While not part of Assad's inner circle, his departure provides a powerful morale boost to the opposition.
Like the rebels, Hijab is from Syria's Sunni Muslim majority, while Assad is from the Alawite minority.
The United States has sought to increase pressure on Assad to leave power for more than a year.
U.S. intelligence officials expect pro-Assad and opposition forces to be in the fight for the long haul and are not predicting a near-term end to the Syrian conflict.
But U.S. officials say the defections are making a dent.
"It stands to reason that mounting defections are taking a psychological toll on the regime," a U.S. official said on condition of anonymity. "There's probably a range of motivations for why people are breaking with Assad, from self-preservation to an inability to tolerate his ruthless tactics."
"The impact of defections on regime stability will depend in part on the role these people play in the opposition," the official said. "And with the heat on the regime rising, Assad may be starting to wonder about the loyalty of some in his inner circle."
Former U.N. Secretary-General Kofi Annan announced last week he would resign his post as U.N.-Arab League Joint Special Envoy for Syria after Assad failed to respect the terms of a U.N.-backed peace deal and after China and Russia refused to support increased sanctions on Syria.
At the White House, Carney said the Obama administration would continue working with other countries and with Syria's opposition to keep pressure on Assad and encourage a peaceful political transition there.