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Foreign ministers from more than 70 Western and Arab countries have sought to increase pressure on Syria at a key meeting in Istanbul.
The "Friends of the Syrian People" summit warned Damascus not to stall on implementing a UN-Arab peace plan and stressed support for the opposition.
However key players remained absent, including Russia, China and Iran.
Damascus dubbed the summit the "enemies of Syria" and has declared its victory over rebel fighters.
'We cannot wait'
Syria has in principle agreed to the six-point peace plan proposed by UN-Arab League envoy Kofi Annan.
However, many of those at the summit appeared sceptical it would implement it.
US Secretary of State Hillary Clinton said: "Nearly a week has gone by, and we have to conclude that the regime is adding to its long list of broken promises."
Opening the summit, Turkish Prime Minister Recep Tayyip Erdogan said: "The Syrian regime should not be allowed at any cost to manipulate this plan to gain time."
But the BBC's Jim Muir, in Beirut, says the Syrian government feels confident that it has little to fear from the Istanbul gathering - state television carried parts of the opening speeches from the conference live, dubbing it the "enemies of Syria" meeting.
Mrs Clinton called for a unified response on renewed action against Damascus should it fail to implement the Annan plan, saying "we cannot sit back and wait any longer".
However, there remain a number of issues on which there appears little unity.
A call by Burhan Ghalioun, leader of the main opposition Syrian National Council (SNC), for "security corridors" inside Syria to allow the delivery of aid to civilians has so far not been heeded.
There is also serious division over arming the rebels. Qatar and Saudi Arabia have argued strongly for such support, but most of those at the summit remain opposed. They fear a flood of weapons could fuel a sectarian civil war.
Mr Ghalioun has also pressed for the SNC to be recognised as "the sole legitimate representative of the Syrian people".
But French Foreign Minister Alain Juppe said the conference had agreed to recognise it as the "main point of contact".
Mr Juppe added that a deadline must be set for Syria to implement the Annan plan.
The head of the Arab League, Nabil al-Arabi, told the summit it should "simultaneously call on the Security Council to take a binding decision... to stop the violence in Syria".
However, Russia and China have balked at Security Council resolutions and were pointedly absent in Istanbul.
Iraq is attending, although it had earlier suggested it might not. However, on Sunday Prime Minister Nouri al-Maliki said he opposed arming the opposition and believed the Syrian government would survive.
He said: "It has been one year and the regime did not fall, and it will not fall, and why should it fall?"
The Syrian government says it is close to ending the uprising.
Syrian foreign ministry spokesman Jihad al-Makdissi told Syrian TV "the battle to topple the state is over".
Violence continued on Sunday, with more than 10 people reported killed, a day after more than 60 people died across the country.
Our correspondent says that in the latest violence, activists reported attacks by security forces in areas near the Iraqi border to the east, and the Jordanian frontier to the south.
The UN believes at least 9,000 people have died in the year-long revolt against Mr Assad's rule.