US Secretary of State Hillary Clinton yesterday warned that time was running out for the Syrian regime and it needed to start a political transition to save the country from a "catastrophic assault."

"It should be abundantly clear to those who support the Assad regime their days are numbered," Clinton told reporters on the sidelines of a conference on Afghanistan in Tokyo.

But she acknowledged that efforts, led by UN-Arab League envoy Kofi Annan, to get the regime of Syrian President Bashar Assad to halt its brutal crackdown on the opposition were proving difficult.

Annan himself in an interview with the French daily Le Monde said his six-point plan to end hostilities in Syria had not been successful so far, and there was no guarantee that it would bear fruit.

Activists say more than 17,000 people have now died since the uprising began in March last year.

"What Kofi Annan said should be a wake-up call to everyone because he acknowledged that there has not been movement by the Syrian regime in accordance with the six-point plan," Clinton said.

Annan arrived in Damascus yesterday, his spokesman Ahmad Fawzi said. It is Annan's third trip to Syria since the outbreak of the conflict, following a previous visit on May 29.

“The Joint Special Envoy for Syria, Kofi Annan, arrived in Damascus this evening for talks with President Bashar Al-Assad,” Ahmad Fawzi said, without elaborating.

Clinton said: “The sooner there can be an end to the violence and a begetting of a political transition process, not only will fewer people die, but there's a chance to save the Syrian state from a catastrophic assault that would be dangerous not only to the country, but the region."

She met with members of the Syrian opposition leading efforts to topple Assad, whose family has ruled the Arab nation for four decades.

"There's no doubt that the opposition is getting more effective in their defense of themselves and is going on the offence against the Syrian military and the Syrian government's militias," Clinton told reporters Sunday.

UN Secretary General Ban Ki-moon, who also attended the donors' conference in Tokyo, renewed his call on the UN Security Council for collective action to pressure Syria to stop the violence.

"It is crucial for the Security Council... to pressure the parties to prevent any further escalation of the conflict," Ban told a separate news briefing Sunday. "President Assad must understand that things cannot continue as they are. Fundamental change is needed," he said.

The UN chief added: "Syrian people have suffered too long and too much. I sincerely hope that the member states of the UN Security Council will look into this issue more seriously ...sharing the common responsibility by taking collective action as soon as possible."

But in an act of defiance, Syria's navy fired live missiles from ships and helicopters over the weekend, state media said yesterday, in an exercise aiming at showcasing its ability to "defend Syria's shores against any possible aggression." Syrian television aired video of a variety of missiles being fired from launchers on land and from ships and showed the Syrian Defense Minister Dawud Abdallah Rahijia in attendance.

"Naval Forces conducted an operational live fire exercise on Saturday, using missiles launched from the sea and coast, helicopters and missile boats, simulating a scenario of repelling a sudden attack from the sea," Syrian news agency SANA said, adding maneuvers would continue for several days.

Opposition figures have been calling for a no-fly zone and NATO strikes against Syrian forces, similar to those carried out in Libya last year which enabled rebel ground forces to end the rule of Muammar Qaddafi.

But while Assad has faced sanctions and international condemnation over his crackdown on dissent which has left thousands dead, major Western and Arab powers have shied away from the idea of direct military action.

More than 35 people were killed yesterday during a government bombardment and clashes between Syrian forces and Free Syrian Army rebels fighting to oust Assad, the Syrian Observatory for Human Rights said.

Activists reported heavy shelling in residential areas of Deir Al-Zor city and in Deraa province, the birthplace of the revolt near the Jordanian border.