US President Barack Obama vowed Saturday to help rescue thousands of civilians besieged by jihadists on an Iraqi mountain, as an MP warned they would not survive much longer.

He gave no timetable for the first US operation in Iraq since the last American troops withdrew three years ago and put the onus on Iraqi politicians to form an inclusive government and turn the tide on jihadist expansion.

"The United States can't just look away. That's not who we are. We're Americans. We act. We lead. And that's what we're going to do on that mountain," Obama said.

The United States has conducted multiple air strikes since Friday, and announced a wave of strikes Saturday it said were to defend attacks on members of the Yazidi minority, who have been stranded on Mount Sinjar since they fled Islamic State attacks on their homes a week ago.

US and Iraqi aircraft have also air dropped food and water to the thousands of people, many of them Yazidi civilians, stranded on the mountain.

France and Britain announced that aid consignments of their own were imminent.

Two Royal Air Force (RAF) C-130 transport planes took off from Britain Saturday carrying reusable filtration containers filled with clean water, tents, tarpaulins and solar lights that can also recharge mobile phones.

Amid reports that the children and elderly among them were already dying, Obama justified the decision to intervene Thursday with the risk of an impending genocide against the Yazidis.

Yazidi MP Vian Dakhil, whose poignant appeal in parliament this week made her the public voice of her community, said time was running out.

"We have one or two days left to help these people. After that they will start dying en masse," she told AFP Saturday .

The Yazidis, who worship a figure many Muslims associate with the devil, are a small and closed community, one of Iraq's most vulnerable minorities.

- Pressure on Maliki -

A first day of US air raids targeted fighters who had moved within striking distance of Kurdistan, while the second day US planes and drones hit armored personnel carriers and an armed truck the Pentagon said were attacking Yazidi civilians near Sinjar.

The first US bombings struck IS positions and at least one convoy of vehicles carrying militants west of Arbil.

The strikes prompted a top Kurdistan official to say the time had come for a fightback -- but there was no immediate sign that was happening.

Security sources and a local official said the bodies of 16 Sunni extremists killed in Makhmur, where IS positions were bombed on Friday and fighting with peshmerga also took place, had been buried nearby on Saturday.

Obama said he had authorised the strikes in Iraq to protect US personnel serving there. "And, if necessary, that's what we will continue to do," he said Saturday.

Federal and Kurdish officials, who had been at loggerheads since IS fighters launched their an offensive exactly two months ago that has brought Iraq to the brink of partition, have said they were now working together and with US advisers.

But it remained unclear how much longer and how much deeper inside Iraq US warplanes would intervene and Obama stressed the real game-changer would be the much-delayed formation of an inclusive government.

Until then, he said, "it is very hard to get a unified effort by Iraqis against" IS.

- Dehydration -

Many Iraqis see Prime Minister Nuri al-Maliki as partly responsible for the conflict by institutionalising sectarianism.

Washington, Tehran, the Shiite religious leadership and much of his own party have pulled their support but he has dug his heels in and apparently not yet given up on seeking a third term.

Grand Ayatollah Ali al-Sistani, Iraq's most revered Shiite cleric, on Saturday alluded to Maliki when he complained "there were some people who do not want the good of the country."

He was being quoted, after a meeting in the city of Najaf, by Chaldean Catholic Patriarch Louis Sako, whose community was displaced on an unprecedented scale this week.

Up to 100,000 Christians were forced to flee from their homes in a matter of hours on Thursday, completely emptying the country's largest Christian city Qaraqosh of its population.

Among the hundreds of thousands of people who fled their homes in northern Iraq were several other minorities such as the Shabak and Turkmen Shiites.

UNESCO chief Irina Bokova called it an "emerging cultural cleansing".

"The US should strike Sinjar, even if there are civilian casualties. It's better than letting everyone die," the Yazidi MP, Vian Dakhil, said.

Obama said he was confident the US could prevent IS fighters "from going up the mountain and slaughtering the people who are there" but added the next step of creating a safe passage was "logistically complicated".

The International Rescue Committee (IRC) is providing emergency care to around 4,000 people who crossed safely into neighbouring Syria.

"They suffer from dehydration, sunstroke and some of them are seriously traumatised," the IRC's Suzanna Tkalec told AFP, adding that many had walked all day for several days.