Two US fighter jets have dropped bombs on Islamic militants towing artillery near American personnel in Iraq.
The Pentagon said two F/A-18 jets dropped 500-pound bombs on a piece of artillery and the truck towing it outside Irbil.
President Barack Obama authorised air strikes when the Islamic State militant group advanced on Irbil, in north-eastern Iraq, where US military trainers are stationed.
In a televised speech last night, Mr Obama had threatened to renew US military involvement in Iraq's long sectarian war. He said American military planes had already carried out air-drops of food and water, at the request of the Iraqi government, to tens of thousands of Iraqi religious minorities atop a mountain surrounded by militants and desperately in need of supplies.
"America is coming to help," the president said in a sombre speech from the White House.
The Yazidis, who follow an ancient religion with ties to Zoroastrianism, fled their homes after the Islamic State group issued an ultimatum to convert to Islam, pay a religious fine, flee their homes or face death.
"Earlier this week, one Iraqi in the area cried to the world, 'There is no-one coming to help'. Well, today, America is coming to help," Mr Obama said. "We're also consulting with other countries - and the United Nations - who have called for action to address this humanitarian crisis."
The announcement reflected the deepest American engagement in Iraq since US troops withdrew in late 2011 after nearly a decade of war. Mr Obama has staked much of his legacy as president on ending what he once called the "dumb war" in Iraq.
Mindful of the public's aversion to another lengthy war, Mr Obama acknowledged that the prospect of a new round of US military action would be a cause for concern among many Americans. He vowed anew not to put American combat troops back on the ground in Iraq and said there was no US military solution to the crisis.
"As commander in chief, I will not allow the United States to be dragged into fighting another war in Iraq," he said.
But, in reference to American troops in the northern city of Irbil and the US consulate there, he added: "When the lives of American citizens are at risk, we will take action. "That's my responsibility as commander in chief."
He addressed the nation only after the American military aircraft delivering food and water to the Iraqis had safely left the drop site in northern Iraq.
The Pentagon said the airdrops were performed by one C-17 and two C-130 cargo aircraft that together delivered a total of 72 bundles of food and water. They were escorted by two F/A-18 fighters from an undisclosed air base in the region.
Administration officials said they believe unilateral US strikes would be consistent with international law in part because the Iraqi government has asked for Washington to take military action. They also said Mr Obama had the constitutional authority to act on his own in order to protect American citizens.