At the Morocco meeting, Burns told Syrian rebel leaders that their newfound recognition is freighted with the weight of international expectations.
"This leadership comes with real responsibilities," he said, according to a transcript posted on the State Department's website.
On top of previous commitments, Burns said the United States will provide $14 million for emergency medical care and for supplies to help Syrians live through the coming winter, including plastic insulation, boots and nutritional supplies.
Saudi Arabia also pledged $100 million in aid, Shaikh said.
Obama's statement Tuesday came as a surprise to Russia, said Foreign Minister Sergey Lavrov.
He said an agreement he had worked out in Geneva, Switzerland, with U.S. Secretary of State Hillary Clinton laid out a path for a negotiated transfer of power, but he said the new coalition's goals call for it to "overturn the regime, dismantle government institutions and refuse dialogue with the Syrian government."
"We inquired with our American partners as to how that conforms with the logic of the Geneva communique, and they told us that the most important thing is to unite the opposition, and its platform can, quote, 'be corrected,'" Lavrov said.
As the diplomatic talks were going on in Morocco, violence continued in Syria.
State TV showed images of an explosion outside the Interior Ministry in Damascus, saying it was one of three bombings that killed five people and injured 23. The London-based Syrian Observatory for Human Rights said the explosion killed eight Syrian soldiers and wounded 40.
The state-run SANA news agency said two bombs exploded behind the Justice Palace, injuring one person.
The opposition Local Coordination Committees of Syria said 113 people had died in fighting Wednesday, including 15 children. The group said government warplanes struck at targets in the suburbs of Damascus as rebels and government forces clashed nationwide.