In one of its strongest statements to date on Syria, the United States says President Bashar al-Assad has lost all legitimacy and credibility.
Opposition groups in Syria say more than 200 people have been killed in the past two days.
The White House spokeperson, Jay Carney, says more and more Syrians are rising up against Mr Assad.
"Defections from the military are on the rise," he says; "diplomats have begun leaving their posts and coming out in support of the Syrian opposition; the opposition is more unified and more exclusive; the regime has been cut off by the Arab League, by its traditional allies and neighbours like Turkey.
"Only fear is holding it together, and governing that is based on fear is always doomed to fail."
The Assad regime insists it is fighting armed terrorist gangs.
Emergency meetings called for
Meanwhile, Syrian opposition leaders have called for emergency meetings at the United Nations and the Arab League to address the worsening violence in the country.
An Arab League team of monitors is due to arrive in the country shortly to begin the month-long task of assessing the situation on the ground.
One activist says however that the Syrian military has already begun concealing some of its equipment, taking armoured weapons out of the country before the monitors arrive.