The United States is exploring options to evacuate thousands of Iraqi civilians trapped on a barren mountain in northern Iraq by Islamic militants, after four nights of humanitarian relief airdrops, U.S. officials said on Sunday, Aug 10, according to Reuters.

"We're reviewing options for removing the remaining civilians off the mountain," the agency quoted deputy U.S. national security adviser Ben Rhodes as saying. "Kurdish forces are helping, and we're talking to the (United Nations) and other international partners about how to bring them to a safe space."

The UN mission in Iraq has also said it is preparing a humanitarian corridor to permit the Yazidis to flee to safety.

The group are followers of an ancient religion derived from Zoroastrianism. They are viewed as "devil worshippers" by the Sunni militants of Islamic State who tell them to convert to Islam or face death.

More than 30,000 Yazidis, mainly from Sinjar, have already crossed into an area of northern Iraq controlled by Kurdish security forces after a week-long journey that took them through Syria after they left the mountain retreat that had become a graveyard for many, according to Yazidis and UN officials.

"That’s going to be a very big operation,' said Ken Pollack, a former CIA and White House expert on the region, now at the private Brookings Institution. "They can't stay on the mountain. They have to leave."

On Sunday night, four U.S. cargo aircraft dropped food and water in the latest delivery, the U.S. military's Central Command said in a statement. U.S. forces have dropped a total of more than 74,000 meals and more than 15,000 gallons of fresh drinking water so far to those trapped on the arid mountain.

Islamic State militants have seized large swathes of northern Iraq since June, breaking out of their original operating areas in nearby Syria.

Rhodes said the airdrops have been effective, and noted that U.S. aircraft have also attacked Islamic State fighters who have laid siege to Sinjar mountain.