US military aircraft dropped humanitarian aid to Iraqis under threat from hardline militants in northern Iraq for the second straight night, the Pentagon said.

"This airdrop was conducted from multiple airbases within the US Central Command area of responsibility and as with last night, included one C-17 and two C-130 cargo aircraft that together dropped a total of 72 bundles of supplies," the Pentagon said in a statement.

The cargo planes were accompanied by two F-18 fighter jets from the USS George H.W. Bush, a US aircraft carrier positioned in the Gulf, it said.

The supplies included 28,224 U.S. military rations and 1,522 gallons of drinking water.

"To date, in coordination with the government of Iraq, U.S. military aircraft have delivered 36,224 meals and 6,822 gallons of fresh drinking water, providing much-needed aid to Iraqis," the Pentagon said.

The Obama administration ordered strikes on fighters from the Islamic State, an al Qaeda offshoot, as it seeks to stop the militants from overrunning Arbil, the capital of Iraq's northern Kurdish enclave, and from continuing attacks on religious and ethnic minorities.