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, US officials said on Sunday.

While the airdrops appear to have provided urgently needed aid, the harsh conditions of the Sinjar mountain range in mid-summer have taken scores of lives among Iraq's Yazidi minority, who are threatened by hardline militants from the Islamic State.

Proposals for a risky mission to save the group underscore the limits of the airdrops, ordered last week by US President Barack Obama.

"We're reviewing options for removing the remaining civilians off the mountain," deputy US national security adviser Ben Rhodes told Reuters. "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.

Also Read: Who are Iraq's Yazidis and why is the Islamic State targeting them?

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.

Yet any mission to evacuate the remaining Yazidis from the mountain is likely to be perilous, and could test Obama's pledge to limit US involvement in Iraq's latest chaos.

"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 US cargo aircraft dropped food and water in the latest delivery, the US military's Central Command said in a statement. US 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.

Safe passage?

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 US aircraft have also attacked Islamic State fighters who have laid siege to Sinjar mountain.

Still, the plight of the Yazidis, which prompted a reluctant Obama to intervene militarily in Iraq last week, remains acute. "They are in dire need of everything. Food, water, non-food items, hygiene and sanitation," said Eliana Nabaa, spokesperson for the UN mission in Iraq.

Pollack said there are just two options for securing safe passage for the Yazidis off the mountain.

One, he said, is for UN representatives to convince Islamic State to let them go or be pummeled by American airstrikes. The second is a corridor secured by Peshmerga or Iraq army troops and US airpower.

To establish a humanitarian corridor, the United Nations and any nations that participated would have to overcome the Islamic State's military advantage over Kurdish security forces known as the Peshmerga.

"Security would have to be provided by the Iraqis, especially the Kurds, with air cover from the US and possibly the British and the French," a UN official said on condition of anonymity.

Obama has insisted that he will not send US combat troops back to Iraq, saying the US military response will be limited to protecting the Yazidis and the Kurdish city of Arbil, where numerous US advisers are present.

For now, many Yazidis appear to prefer contending with the Sinjar mountain than taking their chance with the Islamic State.

Iraqi Human Rights Minister Mohammed Shia al-Sudani said on Sunday Islamic State fighters killed hundreds of Yazidis after seizing Sinjar, burying some alive and taking women as slaves.

Fred Hof, a former senior State Department official now at the Atlantic Council, said the US strikes could help by enabling the Peshmerga, who have suffered recent defeats at Islamic State hands, to regain the advantage.

"The key to rescuing tens of thousands of Yazidis is for the Peshmerga - with tactical air support from US Naval Aviation and Air Force assets - to clear the Sinjar area of (Islamic State) fighters and make it possible to rescue and resettle these terrified people and allow truck loads of emergency humanitarian aid to reach them," he said.