Up to 150,000 desperate refugees were still isolated on a barren stretch of mountain in Iraq, surrounded by Islamic extremists despite a major international intervention.
There were reports that thousands of Yazidi Kurds, including children, may have already lost their lives after being trapped without food and water for days. There were fears the death toll can only rise.
President Barack Obama has committed the US to long-term involvement in Iraq, warning that the rapidly evolving crisis in the north would not be solved quickly.
Conceding that the advance of the Isis (Islamic State) forces had been swifter than anticipated - details emerged yesterday of the jihadists opening another front as they crossed into Lebanon from Syria - Obama accepted there was no quick fix.
The British Government said the refugees faced a desperate choice between risking dying from a lack of food and water in temperatures of up to 50C on the mountainside or "descending into the barbaric hands of terrorists".
A Sunday Telegraph journalist flew on to Mt Sinjar in a relief helicopter.
He watched as hundreds of refugees ran toward the helicopter for one of the few deliveries of aid to make it to the mountain. The helicopter, which was sent by the Iraq Army Aviation force, dipped low, opened its gun bays and dropped water and food into the arms of the waiting refugees.
General Ahmed Ithwany, who led the mission, said: "It is death valley. Up to 70 per cent of them are dead."
American aid flights have also made it to the mountain, where they have dropped off more than 52,000 meals and 40,125 litres of fresh drinking water to help the refugees. Some of these reportedly burst on impact.
Only handfuls of refugees have managed to escape on the helicopters because the craft are unable to land on the rocky mountainside.
The first British aid was dropped from a C-130 transport plane on to the mountainside. The aid included water, tents, solar lighting and purification equipment. France is also sending aid.
But despite the international efforts there were growing concerns that the aid so far delivered would not be enough to stem the growing humanitarian crisis.
British Prime Minister David Cameron said he is working with the US on a plan to help get people off the mountain, amid speculation that British military personnel could become directly involved in the rescue effort. A spokesman for Downing St said: "The long-term solution will involve getting these people to safety."
Obama said yesterday the West could not look away from the plight of "countless innocent people facing a massacre".
He warned that jihadists based in Iraq could mount attacks on Western targets as he admitted intelligence agencies and governments around the world had underestimated Isis.
Obama admitted that rebuilding the Iraqi military, fostering trust among Sunnis and negating the threat from jihadists would be a long-term project. "I don't think we're going to solve this problem in weeks. This is going to take some time."
He insisted, however, that the US would not put troops in Iraq.
However, Lord Dannatt, the former head of the British Army, suggested soldiers should be stationed in northern Iraq to help create a "safe area" for the refugees and said Britain has a "moral obligation" to join the air strikes.
Douglas Alexander, Labour's shadow foreign secretary, said he supported the Government's decision not to mount a military intervention.
Sir Malcolm Rifkind, a former British Foreign Secretary, said: "I was against the Iraq War, it was all a very foolish mistake and has contributed to the chaos in Iraq. At a humanitarian level there is an intervention. What happened 10 years ago should make us more cautious about military intervention in Iraq."
The situation in Iraq intensified yesterday amid reports that Islamic militants have threatened to execute more than 300 Yazidi families unless they convert to Islam.
The Yazidis, a religious group with links to Zoroastrianism, are a Kurdish-speaking minority regarded as "devil worshippers" by Isis.
Isis forces continued to push towards Erbil in Kurdistan, where the US has an embassy, opposed by Kurdish Peshmerga forces.
The militants yesterday hoisted a black flag over Mosul dam, the largest in the country. There were reports that the militants had even sent in their own engineers to start repairs on it.
The archbishop of Erbil's Chaldean Catholics said fewer than 40 Christians remained in northwestern Iraq after a jihadist rampage that has forced thousands to flee from Mosul and the Nineveh plains into Irbil in the Kurdish north.
After taking in up to 1.2 million refugees since mid-June, the Kurds of northern Iraq are urging Obama not to let up in air strikes against Isis, which was only 50km from Erbil. At least four US air strikes appear to have slowed the momentum of the jihadists, Kurdish Peshmerga forces said yesterday.
Refugees wait on mountain
Mt Sinjar rises from a vast expanse of the barren border region of northern Iraq: a near vertical ridge reaching a rocky plateau of baked earth and jagged rocks.
Small groups of men, women, and overwhelmingly children, wave at the sky as a Kurdish helicopter carrying food and water descends, throwing up clouds of dust. As the helicopter lands, a crowd appears on the horizon. A human train of the Yazidi of Sinjar province - persecuted in today's Iraq controlled by Isis (Islamic State) together with Christians of Mosul and Qaraqosh.
Convert or die is the ultimatum issued by Isis to other religions. Those stranded atop Mt Sinjar fled before being killed. Isis considers the Yazidis apostates and has vowed to exterminate them.
Of the tens of thousands of Yazidis stranded on the mountain top, only 20 manage to squeeze into the Kurdish helicopter.
The Yazidis had spent a week on the mountain, reliant on aid deliveries and air drops from the West. Yesterday, many were reported to have sought refuge in another country ravaged by conflict, Syria. Some managed to collect water and food, dropped by US planes, before heading northwest on a 20km walk across mountains and desert to the Syrian border.
At the border, Syrian Kurdish forces waited to take them to refugee camps or safe crossings back into the Kurdish region of Iraq.
Rescue operation at Sinjar mountain
150,000 Yazidi refugees are trapped on Mt Sinjar in 50C temperatures fleeing Islamic State militants
*4 more airstrikes yesterday on Isis militants in Iraq, taking out armoured carriers and a truck that were firing on civilians, the 3rd round of airstrikes against Isis forces since Friday.
*2 US Navy F/A-18 fighter jets had earlier dropped bombs on an Isis artillery unit and the US used a drone to target a Isis mortar position.
*4 jets also destroyed a convoy of 7 Isis vehicles.
*72 bundles of supplies dropped by one C-17 and two C-130 cargo planes escorted by US fighter planes.
*16,000 packaged meals and 14,385 litres of bottled water dropped.
*52,000 meals and 40,125 litres of fresh drinking water delivered so far, the Pentagon said.
*2 British C-130 aircraft were to drop bottled water, tents and tarpaulins and France is also sending an aid drop.
-Telegraph Group Ltd, AP, AFP, Observer, Independant