Thousands of Shiite Turkmen have been stranded in the farming community about 105 miles (170 kilometers) north of Baghdad. The aid came at the request of the Iraqi government, Pentagon press secretary Rear Adm. John Kirby said in a statement.
Military operations will be limited in scope and duration as needed to address the humanitarian crisis in Amirli and protect the civilians trapped in the town, Kirby said.
Instead of fleeing in the face of the Islamic State drive across northern Iraq, the Shiite Turkmens have stayed and fortified their town of 15,000 with trenches and armed positions.
While Amirli fought off the initial attack in June, it has been surrounded by the militants since mid-July. Some residents have said that the Iraqi military's efforts to fly in food, water and other aid have not been enough amid oppressive heat, lack of electrical power - the town's power station was destroyed weeks ago - and shelling from the militants.
The U.S. had been watching the area closely in case a slaughter of the Turkmen appeared imminent and air support was needed, said Michael Knights, who studies Iraq and the Persian Gulf as a fellow of The Washington Institute. U.S. airstrikes will hasten the success of the relief effort on the ground, he said.
About half of the town's population is age 15 and under while many others are elderly, sick or wounded, Knights said.
''They are remarkably vulnerable, and ISIS is determined to kill as many of these people as possible,'' Knights said, referring to an acronym for the Islamic State group. ''As the Nazis felt about the Jews, so ISIS feels about the Shia Muslims.''
U.S. airstrikes in Iraq, which began earlier this month, have targeted Islamic State militants attacking Yazidi Iraqis on Mount Sinjar and the militant forces operating in the vicinity of Ibril and Mosul Dam. The beleaguered Yazidis received several humanitarian drops of tons of food and water as well as military support aimed at protecting them.
Earlier Saturday, U.S. Central Command said five more airstrikes had taken place against Islamic State militants near Mosul Dam. Those attacks, carried out by fighter aircraft and unmanned drones, brought to 115 the total number of airstrikes across Iraq since Aug. 8.