Ahead of a meeting of the Friends of Syria group in Amman, US Secretary of State John Kerry urged Assad to make a "commitment to find peace" after more than two years of conflict that have killed more than 94,000 people.
But he said that "in the event that we can't find that way forward, in the event that the Assad regime is unwilling to negotiate... In good faith, we will also talk about our continued support and our growing support for the opposition to permit them to continue to be able to fight for the freedom of their country."
"It is the longstanding view of the UK that Assad needs to go, and we have never been able to see any solution which involves him staying," British Foreign Secretary William Hague told reporters.
Qatar, a key supporter of the Syrian opposition, echoed that.
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"A political solution must be reached to end the conflict and meet the aspirations of the Syrian people who, as we know, demand changing the regime and changing President Bashar al-Assad, who insists on killing his people," Sheikh Hamad bin Jassem al-Thani said in Doha.
Representatives of the Syrian opposition yesterday demanded international guarantees that Assad would step down as part of any peace deal and have no further stake in Syria's future.
The United States and Russia, which back opposite sides in the conflict, earlier this month proposed a peace conference dubbed Geneva 2 to bring together rebels and representatives of Assad's regime.
The aim of the conference, Hague stressed, would be to agree on the formation of "a transitional government with full executive authority, formed on the basis of mutual consent."
French Foreign Minister Laurent Fabius agreed.
"There are some conditions and in particular conditions about participation, which must be representative and which must not include countries which are against success," he told reporters in Amman, in an apparent allusion to Assad ally Iran.