Hillary Clinton, the US secretary of state, issued the threat at a conference of the Friends of the Syrian people in Istanbul on Sunday, but there was little evidence of coherent international action if he does not comply.
Syria announced last week that it had accepted a six-point plan being promoted by the former UN secretary general Kofi Annan, though Assad then demanded that the "armed terrorist groups" he says are supported by an Arab-western "conspiracy" disarm first. According to the UN, 9,000 people have been killed in Syria over the last year.
"Nearly a week has gone by, and we have to conclude that the regime is adding to its long list of broken promises," Clinton told the representatives of 83 countries as pro-Assad protesters demonstrated outside the conference venue. "The world must judge Assad by what he does, not by what he says. And we cannot sit back and wait any longer."
Russia and China, traditional allies of Assad, stayed away from the meeting, while Syrian officials and media scorned it. Opposition activists, under pressure to close ranks, appeared less divided than before, but many dismissed the event as a display of impotence or even as an April Fool's Day Joke.
Annan's six-point plan calls for a ceasefire, military withdrawals from towns, prisoner releases, humanitarian and media access and above all a "Syrian-led" political process to negotiate transition to a new government.
On Saturday, the Damascus regime claimed victory over its enemies, declaring an end to the 13-month uprising and the start of a new stage of "stabilisation".
But the repeated calls in Istanbul to adopt the Annan plan sounded half-hearted. Annan, who is due to brief the UN security council on Monday, did not even attend. Diplomats suggested he would have to set a deadline for implementation. And the signs are that several countries are already preparing for the failure of the initiative.
Turkey's prime minister, Recep Tayip Erdogan, said Syrians had the right to defend themselves if the UN was unable to resolve the crisis. William Hague, Britain's foreign secretary, described the situation in Syria as dire and warned that calls to arm the opposition would increase if there was no progress. Saudi Arabia said on Saturday it was a duty to do so – in the face of warnings from the US and others that that would make a bad situation worse.
"We're working on co-ordinating our sanctions together and sending a clear message that there isn't an unlimited period of time for this, for the Kofi Annan process to work before many of the nations here want us to go back to the UN security council," Hague told the BBC.
But even a return to the security council would not guarantee a change of attitude by Russia and China.
Practical steps to emerge from Istanbul included channeling Gulf cash to the Syrian National Council to pay the fighters of the Free Syrian Army, the armed wing of the opposition. Payments would also be made to those who dared to defect from the Assad regime, whose senior ranks have so far remained solid.
Divisions in the opposition were less obvious than during the last Friends of Syria meeting in Tunis, but there was a yawning gap between rhetoric and reality. "We demand serious action," urged Burham Ghalioun, chairman of the SNC. "The Syrian regime will inevitably fall. Don't prolong the catastrophe." But there was no response to his call for the creation of "humanitarian corridors" to channel aid into the country because it smacks of the military intervention few outsiders are prepared to risk.
Building the Syrian State, an opposition group, said it rejected "absolutely" any decision that would fuel the conflict, including arming the rebels. Other opposition supporters reacted with open derision to a conference they believed would do nothing to advance the anti-Assad cause. "#TakeAssadOut & spare us your useless meetings," tweeted one activist.
In one practical step, the conference announced the creation of a Syria accountability centre to document human rights abuses and prepare future prosecutions by international or post-Assad Syrian courts. The UK has taken the lead in this area, seeking to learn lessons from conflicts elsewhere.
Underlining scepticism about the prospect for slow-moving diplomacy by an international community wracked by deep divisions, violence on the ground continued unabated. Activists reported 43 killed on Saturday.
On Sunday, clashes were reported near Syria's borders with Jordan and Iraq. Five rebels, four soldiers and a civilian were killed in clashes in the Deir al-Zor area, according to the Syrian Observatory for Human Rights. Defectors also attacked an army convoy in Idlib province, killing at least four soldiers and wounding 11. Another 21 civilians, rebels and soldiers died elsewhere, including in Deraa and Homs.
