WASHINGTON: 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 it repeated its 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 United States has been seeking to increase pressure on Assad to leave power for more than a year.

Former UN Secretary-General Kofi Annan announced last week he would resign his post as UN-Arab League Joint Special Envoy for Syria after Assad failed to respect the terms of a UN-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.