Two U.S. aircraft on Friday bombed Islamic State extremist positions in northern Iraq after artillery fire near U.S. personnel, the Pentagon said.
The raid -- the most significant since the United States withdrew from Iraq -- came a day after President Barack Obama authorized force as he voiced fears of genocide against minorities.
Two U.S. F/A-18 aircraft dropped 500-pound (225-kilogram) laser-guided bombs on a mobile artillery piece near the Kurdish region's capital of Arbil, said Pentagon spokesman Rear Admiral John Kirby.
The United States bombed the position after Islamist fighters shelled Kurdish forces defending Arbil, where U.S. personnel are stationed, he said.
The strike, carried out at 10:45 GMT, targeted the Sunni Muslim extremist movement Islamic State of Iraq and the Levant that has swept across Syria and Iraq.
"As the president made clear, the United States military will continue to take direct action against ISIL when they threaten our personnel and facilities," Kirby said in a statement.
President Barack Obama has ordered U.S. warplanes back into Iraqi skies to stop jihadists from moving into autonomous Kurdistan and carrying out a potential genocide against displaced minorities.
Obama, who opposed the 2003 invasion and has vowed no return of ground troops, said Thursday that he authorized action due to fears of genocide as thousands of members of the Yazidi minority fled for their lives.
On Thursday, the United States dropped thousands of gallons of drinking water and 8,000 packaged meals to Yazidis who risk starvation as they cram onto Mount Sinjar.
Many people who have been cowering in the Sinjar mountains for five days in searing heat and with no supplies are Yazidis, a minority that follows a 4,000-year-old faith.
Obama accused the IS, which calls Yazidis "devil-worshippers," of attempting "the systematic destruction of the entire people, which would constitute genocide."
He also justified possible air strikes because of the jihadist threat to Washington's Kurdish allies, following a lightning advance that saw the Sunni extremists move within striking distance of Arbil.
"We plan to stand vigilant and take action if they threaten our facilities anywhere in Iraq, including the consulate in Arbil and embassy in Baghdad," Obama said.
Obama's announcement came after an emergency UN Security Council meeting called by France, which also offered to support forces battling the jihadists.