Armed residents of Amerli have managed to fend off attacks by the Islamic State fighters, who regard its majority Shi'ite Turkmen population as apostates. More than 15,000 people remain trapped inside.
"At the request of the government of Iraq, the United States military today airdropped humanitarian aid to the town of Amerli, home to thousands of Shia Turkmen who have been cut off from receiving food, water, and medical supplies for two months by ISIL," Rear Admiral John Kirby, the Pentagon press secretary, said, using an alternative name for Islamic State.
"In conjunction with this airdrop, U.S. aircraft conducted coordinated air strikes against nearby ISIL terrorists in order to support this humanitarian assistance operation," he said, adding that a key objective was to prevent a militant attack on civilians in the town.
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He said the operations would be "limited in their scope and duration" as required to protect Amerli's population.
When Obama ordered the first air strikes and air drops in Iraq earlier this month, he justified the military operation in part to prevent a humanitarian catastrophe for thousands of ethnic Yazidis trapped by Islamic State militants on Sinjar mountain in northern Iraq.
In mid-August, he declared that the militant siege there had been broken.
Earlier on Saturday, two officers said Iraqi troops, militia and Kurdish peshmerga were advancing on Amerli from four directions.
A major in the Iraqi army, who was advancing north towards Amerli from Udhaim, said progress was slow because the militants had mined the roads. He said they were around 15 kilometers (9 miles) from the town, while those approaching from the north were just 3 kilometers away.
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The major said he had counted the corpses of more than 40 militants killed in Iraqi air strikes on the road between Udhaim and the village of Injana.
Also on Saturday, the Pentagon said U.S. warplanes and armed drones had carried out five air strikes on Islamic State fighters near Iraq's largest dam, the latest in a series of attacks in support of Iraqi and Kurdish forces.