US secretary of state Hillary Clinton warned hardline Syrian leader Bashar al-Assad that his days were numbered as both countries intensified the rhetoric in a war of words on Sunday.
Speaking in Tokyo, Clinton said an admission by UN mediator Kofi Annan that his peace plan was failing should serve as a "wake-up call for everyone". As to the future of Assad, the state secretary said it should be clear that "the sand is running out of the hourglass".
Meanwhile the Syrian president accused Washington on Sunday of fuelling the bloody uprising in his country by partnering with "terrorists".
In an interview with German public broadcaster ARD, Assad said the US had aided rebels "with weapons, money or public and political support at the United Nations".
He also vowed that he would not step down "in the face of national challenges".
The comments came a day after Annan told French daily Le Monde that divisions between regional and world powers over how to resolve the conflict had complicated attempts to forge a settlement.
"The evidence shows that we have not succeeded," he said in reference to his peace plan.
In Tokyo, Clinton said that the worsening situation in Syria risked pushing the country to the point of collapse.
The state secretary noted that last months was the bloodiest of the conflict so far.
But she added that there was "still a chance to save the Syrian state from a catastrophic assault that would be very dangerous not only to Syria, but to the region".
Escalating violence in Syria has so far claimed in the region of 15,000 lives as rebels seek to topple the repressive Assad regime.
Clinton has accused China and Russia of hampering international efforts for a solution, calling on both countries to "get off the sidelines" and back Assad's ouster.
Meanwhile, Annan arrived in the Syrian capital of Damascus on Sunday for talks with the Syrian leader.
His six-point peace plan was due to begin with a ceasefire in mid-April. But it never took hold, with government forces continuing their assault on rebel strongholds.
In her comment Sunday, Clinton noted that the opposition was "getting more effective in defence of themselves and going on the offensive against the Syrian military".
Throughout the weekend, Assad's army were engaged in large-scale exercises, simulating defence manoeuvres against outside "aggression", state media said in an apparent warning to Washington.
