BEIRUT— Syria’s main exile opposition coalition elected a naturalized Syrian-born American citizen early Tuesday to be the first prime minister of an interim Syrian government, charged with funneling aid to rebels inside Syria and offering an alternative to the government of President Bashar al-Assad. By choosing Ghassan Hitto, 50, an information technology executive who lived in Texas until recently, the Syrian opposition coalition concluded months of contentious efforts to unite behind a leader, under pressure from the United States and its allies, which demanded that the opposition set up clear chains of command as a condition of increasing aid to the rebels. Mr. Hitto, a relative unknown in opposition politics who rose to prominence recently through efforts to improve the delivery of humanitarian aid, was far from a unanimous choice. After a day of maneuvering and voting on Monday that lasted into early Tuesday, he won 35 votes, just three more than Assad Mustafa, a former agricultural minister under Mr. Assad’s father and predecessor, Hafez al-Assad. Mr. Hitto faces formidable challenges in his quest to to establish administrative authority over areas of northern Syria that have been secured by the rebels. Mr. Assad’s air force still rules the skies, so any attempt to govern from those rebel-held areas risks the constant threat of airstrikes. And antigovernment fighters and activists inside Syria, who have long complained that the coalition offered little concrete help and had little connection to the struggle on the ground, remain skeptical of any interim government based outside the country. Even opposition leaders outside Syria are divided on whether an interim government makes sense. Fahed al-Masri, a spokesman for the rebel Free Syrian Army’s unified command, questioned how a government could function when it controlled little territory or money yet would be held responsible for the fate of more than one million Syrian refugees and several times that number displaced inside the country.