International envoy Kofi Annan arrived in Syria, his spokesman said, after admitting that his peace plan has so far failed to end nearly 16 months of carnage.
"The Joint Special Envoy for Syria, Kofi Annan, arrived in Damascus this evening for talks with President Bashar Al-Assad," his spokesman Ahmad Fawzi said of Annan's third trip to Syria since the outbreak of the conflict.
Earlier, Syrian foreign ministry spokesman Jihad Makdissi had told AFP Annan would visit for talks on his six-point plan for peace with the Syrian leadership.
US Secretary of State Hillary Clinton warned on Sunday that time was running out to save Syria from a "catastrophic assault," and as dozens more people were reportedly killed in the unrest-swept country.
Annan himself has said his UN-backed mission has so far failed to halt the bloodshed, while stressing that Russia and Iran must not be sidelined from peace efforts.
"Russia wields influence but I am not sure that the events will be determined by Russia alone... Iran is an actor. It has to be part of the solution. It has influence and we cannot ignore it," Annan told France's Le Monde daily.
He also expressed irritation that while Russia and Iran were mentioned by some as stumbling blocks to peace, "little is said about other countries which send arms, money, and have a presence on the ground."
Iranian President Mahmoud Ahmadinejad has accused the United States and its allies of opposing Assad's regime with the goal of dominating the Middle East and propping up Israel.
Iran's deputy foreign minister Hossein Amir Abdollahian said on Sunday in Jordan that any attack on Syria would be "stupid" and "catastrophic."
"Military action in Syria is unlikely and if this happens it would be stupid. Syria can defend itself without Iran's help. Any non-political solution would bring catastrophe to the entire region," he said.
Syria's state news agency SANA said on Sunday that the country's navy staged live fire exercises to "simulate the scenario of repelling a sudden attack from the sea."
Earlier, Clinton acknowledged in Tokyo that efforts led by Annan to get Assad's regime to halt its crackdown were proving difficult.
"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 told reporters.
"It should be abundantly clear to those who support the Assad regime their days are numbered."
Clinton was speaking after talks in Paris on Friday where countries pledged to increase pressure on Assad to step down by seeking a tough UN resolution backed by a threat of sanctions.
UN Secretary General Ban Ki-moon, also speaking in Tokyo, renewed his call on the Security Council for collective action to pressure Syria to stop the violence.
"President Assad must understand that things cannot continue as they are. Fundamental change is needed," Ban said.
But Assad remained defiant.
The United States is "part of the conflict. They offer the umbrella and political support to those gangs to... destabilise Syria," he told German public broadcaster ARD in an interview to be broadcast later on Sunday.
Assad said the Annan plan had failed to stop bloodshed because "many countries don't want it to succeed."
"So they offer political support and they still send armaments and send money to terrorists in Syria. They want it to fail in this way," he said.
Republican US Senator John McCain on Sunday took President Barack Obama's administration to task for what he called its "shameful and disgraceful" response to the carnage in Syria.
"The fact is that the United States has played no leadership role," McCain told CBS television, referring to efforts to halt Syria's crackdown.
"The United States of America's performance so far has been shameful and disgraceful."
More than 17,000 people have now died since the uprising began in March last year, according to the Britain-based Syrian Observatory for Human Rights.
On Sunday 99 people, including 61 civilians, were killed in violence across Syria, said the Observatory, which gathers its information from a network of activists and witnesses.
The death toll also included 36 regular army troops and two deserters.
Syrian forces attempted to storm the rebel strongholds of Qusayr and Rastan in the central province of Homs, the watchdog said.
Both towns have been outside regime control for months and are rebel strongholds.
Regime forces also shelled villages in the eastern province of Deir Ezzor, killing three people including two children, as well as Aazaz town in the northern province of Aleppo, where three people were wounded, the Observatory said.
Sunday's violence followed another bloody day in which 77 people were killed, mostly civilians, the watchdog said.
EU foreign policy chief Catherine Ashton's office released a statement saying she "strongly condemns the recent shelling of the Lebanese border area by Syrian artillery, causing several deaths and injuries."
Rocket fire along the border on Saturday left two girls dead and 10 other people wounded in northern Lebanon.