US jet fighters and drones destroyed armored carriers and a truck that were firing on members of the Yazidi sect, officials said.
Thousands of civilians fled into the mountains after the Islamic State (IS) overran the town of Sinjar a week ago.
IS has taken control of swathes of Iraq and Syria in the past few months.
IS (formerly known as ISIS) has declared a “caliphate”, or Islamic state, in the region, prompting thousands of religious minorities to flee their homes in northern Iraq.
President Barack Obama authorized the military offensive last week to halt the advance of IS forces threatening the Kurdish city of Irbil.
The series of strikes is the first time US forces have been directly involved in a military operation in Iraq since they withdrew from the country in late 2011.
A US military statement said the latest four strikes had been aimed at defending members of the Yazidi religious group who were being “indiscriminately attacked” near Sinjar.
IS has been widely accused of targeting and killing members of other faiths.
The US said a mix of fighter jets and drones destroyed an IS armored personnel carrier (APC) that was firing on civilians.
The statement said US aircraft also attacked other APCs and an armed truck.
The Pentagon also said a third US air-drop of food and water had been made on Saturday night to refugees on Mount Sinjar.
One C-17 and two C-130 cargo aircraft dropped a total of 72 bundles of supplies.
France and Britain have also announced that they will deliver aid consignments.
French Foreign Minister Laurent Fabius is travelling to Baghdad and Irbil for talks on Sunday.
The UN children’s agency, UNICEF, says at least 56 Yazidi children have died of dehydration in the mountains around Sinjar.
Juan Mohammed, a local government spokesman in the Syrian city of Qamishli, told AP news agency that more than 20,000 starving Yazidis had fled across the border.
He said columns of refugees were running a gauntlet of gunfire through a tenuous “safe passage” being defended by forces of Iraq’s autonomous Kurdistan Region.
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