Secretary of State John Kerry said he regretted al-Khatib's resignation, but said it won't affect U.S. aid to the Coalition.
Speaking to reporters during an unannounced trip to Baghdad, Kerry also said he had confronted Iraq, Syria's eastern neighbor, about allowing Iran access to its airspace for flights the U.S. believes are ferrying in weapons and fighters to the Assad regime.
In a small victory for the opposition, senior Arab diplomats said they would transfer Syria's seat at the Arab League to the Coalition. The Syrian government's membership was suspended earlier in the crisis. The Coalition said it would send a delegation to a league summit that begins Tuesday in Qatar.
The Syrian government, which contends the civil war is an international conspiracy being carried out by terrorists to weaken Syria, did not comment on the Coalition developments. Instead, it hosted a "National Dialogue Forum" in Damascus that included none of the forces seeking Assad's ouster.
Few of the rebels inside Syria paid any attention to the exile opposition's problems, saying the Coalition had never done much for them anyway.
"All this stuff that happens outside never makes any difference to us," rebel fighter Firas Filefleh said via Skype from the northern province of Idlib. He said he and his colleagues respect al-Khatib as a religious figure but that he and the Coalition were ineffective.
"The Coalition has never made any difference for the fighting brigades," he said. "They brought some flour and some canned goods but have never done more than that."
Filefleh said he had no opinion of Hitto and said he had never heard of Gen. Idris, who purports to be the rebels' highest military leader.
Late Sunday, the Coalition circulated videos it said showed Hitto during his first visit to Syria since his election. The videos showed Hitto in a sport coat and jeans, shaking hands in an unnamed town in Aleppo province.
Meanwhile, rebels tried to advance their campaign to gain ground along the southern border with Jordan.
Since last summer, the opposition has seized large swathes of land near the Turkish and Iraqi borders to the north and east, and has used them to organize and build supply lines.
Victory in the south could allow them to do the same there. They have recently seized army checkpoints along a 15-mile (25-kilometer) strip of the border. The Britain-based Syrian Observatory for Human Rights said rebels clashed Sunday with forces at a checkpoint and military base in the area.
Also Sunday, Israel's military said soldiers on patrol in the Golan Heights were fired upon and responded by firing back into Syria. It did not say if the Syrian fire was from rebels or the government.
Rebels have been making gains on the Syrian side of the Golan Heights, a strategic plateau that Israel captured from Syria in the 1967 Mideast war and later annexed.
The U.N. says more than 70,000 people have been killed since Syria's crisis began in March 2011.