(Recasts throughout with U.S. air strikes, aid drops)
By Raheem Salman and Matt Spetalnick
BAGHDAD/WASHINGTON, Aug 30 (Reuters) - The United States carried out air strikes on Saturday against Islamic State fighters near the besieged Shi'ite town of Amerli in northern Iraq and airdropped humanitarian aid to civilians trapped there, the Pentagon said.
President Barack Obama authorized the new military action, broadening U.S. operations in Iraq amid an international outcry over the threat to Amerli's mostly ethnic Turkmen population.
Aid was delivered by U.S. aircraft as well as planes from Britain, France and Australia, signaling headway in Obama's efforts to draw allies into the fight against Islamic State.
Iraqi army and Kurdish forces closed in on Islamic State fighters on Saturday in a push to break the Sunni militants' siege of Amerli, which has been surrounded by the militants for more than two months.
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.
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 km (9 miles) from the town, while those approaching from the north were just 3 km away.
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.
The strikes destroyed an Islamic State armed vehicle, a fighting position and weapons, and damaged a building near Mosul Dam, the Pentagon said. Backed by U.S. air power, Kurdish forces recaptured the strategic facility nearly two weeks ago.
Separately on Saturday, a suicide bomber driving a car packed with explosives killed at least 11 people in a town just south of Baghdad.
Islamic State militants overran most of Sunni Arab areas of Iraq after seizing the northern city of Mosul on June 10, and have proclaimed a caliphate straddling the border with Syria, where they also control vast swaths of territory.
The lightning offensive brought the militants within range of the capital of Iraq's autonomous Kurdistan region earlier this month, prompting air strikes by the United States.
The Kurds have since been slowly regaining ground from the militants and on Saturday advanced on the northern town of Zumar.
Peshmerga spokesman Halgurd Hikmat said control over Zumar would help the Kurds retake Rabia and Sinjar, two other areas seized by Islamic State.
Violence in Iraq this year has reached levels not seen since 2006-2007, when the country was in the throes of civil war.
The suicide bombing took place at a checkpoint at a northern entrance to the town of Yusifiya, 15 km from the capital, said a police officer, who spoke on condition of anonymity.
"The suicide bomber drove into the checkpoint and blew up his car amongst vehicles waiting to be searched," the officer said. (Additional reporting by Isabel Coles in Baghdad; Writing by Matt Spetalnick and Isabel Coles; Editing by Andrew Roche, Toni Reinhold and Paul Tait)