President Barack Obama authorized targeted airstrikes and emergency assistance missions in northern Iraq, saying Thursday the U.S. must act to protect American personnel and prevent a humanitarian catastrophe in the face of advances by violent Islamist militants.

The U.S. military said it completed a delivery of meals and water to thousands of members of a religious minority who fled the town of Sinjar and are trapped in nearby mountains by the group calling itself the Islamic State.

Mr. Obama said he ordered the use of U.S. airstrikes if necessary either to stop militants from closing in on the northern city of Erbil or to allow local forces to aid the Yazidis, the religious minority. No U.S. strikes had been conducted by late Thursday, officials said.

His remarks at the White House capped a day of soaring concern about militant advances in Iraq, where extremist fighters seized control of areas long considered safe and took over the Mosul Dam, the country's largest, according to local reports.

But Mr. Obama also acknowledged domestic jitters about renewed military involvement in Iraq, where America fought an eight-year war.