But Maj. Brad Leighton, a U.S. military spokesman, said U.S. and Iraqi special operations forces had identified snipers on several roofs before the strike was ordered.
An AC-130 gunship then opened fire on enemy positions on three roofs.
"Initial reports indicate 16 criminal fighters were killed," he said in an e-mail response to a query by The Associated Press.
The American support occurred as Iraqi troops struggled against strong resistance from militia fighters in Basra, where Prime Minister Nouri al-Maliki has vowed to keep up the fight despite mounting anger among followers of radical Shiite cleric Muqtada al-Sadr.
The government crackdown has prompted retaliation elsewhere in Shiite areas in Baghdad and other cities in the oil-rich south.
American jets were first called to attack militia positions in Basra on Friday, four days after al-Maliki launched the operation to clear the city of militia violence.
The airstrike followed fierce clashes between the Iraqi security forces and Shiite militiamen, Leighton said in the first confirmation of the airstrikes by the U.S. military.
Iraqi ground forces also killed four suspected militants after coming under unrelenting fire by small arms and rocket-propelled grenades during a raid in a known criminal stronghold in western Basra, Leighton said.
Two women and five children were found unharmed in the targeted building, according to the statement. It added that two more extremists were killed after the Iraqi troops came under attack again from surrounding buildings.
During the gunbattle, Iraqi commandos "and a supporting U.S. special forces team identified additional armed criminal elements on several rooftops in the area," and called in the airstrike, Leighton said.
U.S. jets also dropped two precision-guided bombs later Saturday on a suspected militia stronghold at Qarmat Ali north of the city, British military spokesman Maj. Tom Holloway said.
"My understanding was that this was a building that had people who were shooting back at Iraqi ground forces," Holloway said.
Leighton said he had no further information on that airstrike.