The third meeting of the Friends of Syria group is taking place in Paris.
US Secretary of State Hillary Clinton and Britain's William Hague are among the 50 foreign ministers and delegates.
Assad's UN allies, Russia and China, are boycotting the meeting because they say it is one-sided, while Iran has not been invited.
Officials hope these talks will help endorse recent transition planning by the Syrian opposition and lead to UN Security Council discussions on economic sanctions against Assad's government.
French President Francois Hollande called for stiffer sanctions against Syria's Bashar al-Assad and more support for rebels seeking to drive him from power: "Bashar al-Assad must go... It is in the interest of Syria, of its neighbours and everybody who wants peace in the region," he said at the meeting.
Meanwhile, it was reported that a general who is a personal friend of Bashar al-Assad has defected.
A source in the exiled opposition said Manaf Tlas, a brigade commander in Assad's Republican Guard, is on his way to Paris.
If he throws his support behind the opposition, Tlas - who attended military college with Assad - would be the closest member of the Syrian leader's inner circle to switch sides during a 16-month uprising.