The United States said Wednesday its troops found "far fewer" Yazidi refugees marooned on a northern Iraqi mountain than expected, making an evacuation mission less likely, after air strikes pummeled besieging Islamic militants.

The UN refugee agency has said tens of thousands of civilians, many of them members of the Yazidi religious minority, remain trapped on Mount Sinjar by jihadists from the so-called Islamic State (IS), which has overrun large swathes of Iraq and Syria in a lightning and brutal offensive.

But the Pentagon said that -- based on a firsthand assessment by a small party of US troops -- the plight of those on the mountain was better than feared, and an evacuation mission "is far less likely".

A US military official said the special forces soldiers had returned safely to base at Arbil, the capital of Iraq's autonomous Kurdish region.

Pentagon press secretary Rear Admiral John Kirby said that the troops, which consisted of less than 20 personnel, did not engage in any combat.

"The team has assessed that there are far fewer Yazidis on Mount Sinjar than previously feared, in part because of the success of humanitarian air drops, air strikes on ISIL targets, the efforts of the [Kurdish] Peshmerga and the ability of thousands of Yazidis to evacuate from the mountain each night over the last several days.

"The Yazidis who remain are in better condition than previously believed and continue to have access to the food and water that we have dropped."

Iraqi helicopters and Kurdish troops have been trying to come to the aid of the Yazidi religious minority, and Washington and its allies have been studying ways to airlift them off Sinjar or open a humanitarian corridor.

Various countries are ramping up their efforts to aid the trapped civilians and Kurdish forces battling the militants, and the US has launched a series of air strikes since Friday.

But a US military deployment on the mountain itself would take American involvement to another level.

- 'We went from hunger to hunger' -

Thousands of people have poured across a border bridge into camps in Iraq's autonomous Kurdish region after trekking through neighbouring Syria to find refuge, most with nothing but the clothes on their backs.

Some women carried exhausted children, weeping as they reached the relative safety of Iraqi Kurdistan.

But large numbers of people, including the most vulnerable, remain trapped on Mount Sinjar, said Mahmud Bakr, 45.

"My father Khalaf is 70 years old -- he cannot make this journey," he told AFP as he crossed back into Iraq.

UN minority rights expert Rita Izsak has warned the civilians face "a mass atrocity and potential genocide within days or hours".

For those who managed to escape the siege, the relief of reaching relative safety was tempered by the spartan conditions of the camps hurriedly erected by the Iraqi Kurdish authorities to accommodate them.

"We were besieged for 10 days in the mountain. The whole world is talking about us but we did not get any real help," said Khodr Hussein. "We went from hunger in Sinjar to hunger in this camp."

As the international outcry over the plight of the Yazidis mounted, Western governments pledged to step up help for those still trapped, and the United Nations declared a Level 3 emergency in Iraq, allowing it to speed up its response.

The US said Wednesday it had conducted a seventh airdrop of food and water for those remaining on the mountain, bringing the total aid delivered to the stranded Yazidis in coordination with the Iraqi government to more than 114,000 meals and 35,000 gallons of drinking water.

British Prime Minister David Cameron said Wednesday an international plan was under way to rescue trapped civilians.

He declined to give details of the operation, but said Britain would play a role.

Washington has already said it will ship weapons to the Kurds to help them fight back against the jihadists, and France has followed suit.

- Top cleric opposes Maliki bid -

Washington meanwhile urged Iraqi prime minister designate Haidar al-Abadi to move swiftly to form a broad-based government able to unite Iraqis in the fight against the IS insurgents who have overrun large parts of the country.

Abadi, whose nomination was accepted by President Fuad Masum on Monday, has 30 days to build a team that will face the daunting task of defusing sectarian tensions and, in the words of US President Barack Obama, convincing the Sunni Arab minority that IS "is not the only game in town".

The office of top Shiite cleric Grand Ayatollah Ali al-Sistani released Wednesday a July letter in which he called for incumbent premier Nuri al-Maliki to be replaced.

Maliki has defied growing international pressure to step aside and insisted it would take a federal court ruling for him to quit.

Sistani is revered by millions and has enormous influence among Iraq's Shiite Arab majority.

But even before the release of the Sistani letter, analysts said Maliki had lost too much backing to stay in power.

International support has poured in for Abadi, most importantly from Tehran and Washington, the two main foreign powerbrokers in Iraq.

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