Britain, the US and Turkey have warned Bashar al-Assad that he is running out of time and that the international community is rapidly losing patience with his regime's failure to end the violence in Syria.
Dozens of countries are meeting in Istanbul on Sunday to push for tighter sanctions and increased diplomatic pressure to further isolate Assad – and to urge the opposition to offer a democratic alternative to his rule.
However, the show of solidarity at the Friends of the Syrian People conference has been undermined by the absence of China, Russia and Iran, who disagree with western and Arab allies over how to stop the bloodshed. A peace plan by the UN/Arab League envoy Kofi Annan has so far failed to take hold amid fresh reports of deadly violence.
The Turkish prime minister, Recep Tayyip Erdogan, said the Syrian regime should not be allowed to "manipulate" the plan to win time, indicating that military options might have to be considered if Damascus does not co-operate with Annan's plan and the UN security council fails to unite in opposition to Assad. Russia and China vetoed a UN censure of Assad, fearing the measures could lead to foreign military intervention.
"If the UN security council fails once again to bring about its historic responsibility, there will be no other choice than to support the Syrian people's right to self-defence," Erdogan said.
Hillary Clinton, the US secretary of state, also expressed scepticism that the Syrian government would observe Annan's plans, which call for an immediate ceasefire and a Syrian-led negotiation process.
"Nearly a week has gone by, and we have to conclude that the regime is adding to its long list of broken promises," she said. "The world must judge Assad by what he does, not by what he says. And we cannot sit back and wait any longer."
Clinton urged unity behind a plan that includes more sanctions, humanitarian aid, support for the opposition and the promise of justice one day for regime figures involved in atrocities. She said the US is providing communications equipment to help opposition members in Syria organise, remain in contact with the outside world and evade regime attacks.
The foreign secretary, William Hague, said the issue could return to the security council if current efforts to resolve the crisis fail. "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 – some of them will call for arming the opposition if there isn't progress made," he told BBC1's Andrew Marr show.
"What is now being put to them is a plan from Kofi Annan supported by the whole United Nations security council, and this is an important point, it's supported by Russia and by China as well."
Burhan Ghalioun, leader of the opposition Syrian National Council, called for additional measures, including the strengthening of Syrian rebel forces as well as "security corridors" inside Syria, an apparent reference to the foreign military intervention that the nations meeting in Istanbul have so far been reluctant to support.
"No one should allow this regime to feel at ease or to feel stronger by giving them a longer manoeuvring area," he said. "It's enough that the international community has flirted with the regime in Syria. Something has to change."
The Syrian government launched a pre-emptive attack on the conference, with a front-page editorial in the official al-Baath newspaper calling it a "regional and international scramble to search for ways to kill more Syrians, sabotage their society and state and move toward the broad objective of weakening Syria". The regime has consistently dismissed the country's year-long uprising as a foreign-engineered conspiracy.
The one-day Istanbul meeting follows an inaugural forum in Tunisia in February. Since then, Syrian opposition figures have tried to convince international sponsors that they can overcome their differences.
Tens of thousands of Syrian refugees have fled to neighbouring Turkey, Lebanon and Jordan – leading Turkey to float the idea of a buffer zone inside Syria should the flow of displaced people into its territory become overwhelming. There are concerns that foreign intervention – even if it has a humanitarian goal – could widen the conflict by dragging in other countries and triggering a surge in sectarian tensions.
Some Gulf countries want to arm the Syrian rebels, though there is uncertainty about the composition of rebel groups and their lack of cohesion. The US says military force against the Syrian government is a last resort.
