Homs, Syria's third-largest city, has been under a fierce government attack for nearly three weeks. Government troops continued to shell rebel-held areas in central Syria on Friday, killing at least four people, activists said.
To spur negotiations, the Arab League and United Nations on Thursday jointly appointed Annan, the former U.N. secretary-general, to be their special envoy to Syria with a mandate to bring an end to the violence and promote a peaceful political solution.
Annan will work on bringing an end to "all violence and human rights violations, and promoting a peaceful solution to the Syrian crisis," the two bodies said in a statement.
American officials accompanying Clinton to the Friends of Syria meeting said the group would make clear to Assad that his regime has a moral obligation to end the shelling of civilian areas and allow assistance into the country. The burden is on Assad to respond to the demands of the international community, they said.
"The efforts that we are undertaking with the international community ... are intended to demonstrate the Assad regime's deepening isolation," Clinton said. "Our immediate focus is on increasing the pressure. We have got to find ways of getting food, medicine and other humanitarian assistance into affected areas. This takes time and it takes a lot of diplomacy."
Clinton met Thursday in London with Juppe and foreign ministers and senior officials from about a dozen countries, including Britain, Germany, Jordan, Morocco, Qatar, Saudi Arabia, Turkey, Tunisia and the United Arab Emirates. More than 70 nations and international organizations are expected at the Tunis meeting.
Several nations have proposed creating protected corridors through which humanitarian relief could flow but it was not clear whether a consensus could be reached on the matter, as such a step almost certainly would require a military component.