The United States has carried out its first air strikes on Islamic State extremists in northern Iraq as Britain prepared to launch air drops to assist starving refugees forced to flee the militants.
The Pentagon said US warplanes hit Islamic State artillery positions which had been firing on Kurdish forces defending the city of Irbil, close to where American personnel are based.
Following a meeting in Whitehall of the Government's Cobra emergencies committee, Defence Secretary Michael Fallon said Britain was ready to provide "technical assistance" to support US humanitarian operations in the region.
Speaking in Downing Street, he told reporters that he hoped British air-drops targeting members of the Yazidi religious minority trapped on a mountainside could begin in the "next couple of days".
"What we have decided today is to assist the United States in the humanitarian operations that started yesterday. We are offering technical assistance in that in terms of refuelling and surveillance," he said.
"We are offering aid of our own which we hope to drop over the next couple of days in support of the American relief effort, particularly to help the plight of those who are trapped on the mountain."
In its latest travel advice, the Foreign Office is warning British nationals against all travel to areas in northern Iraq affected by the fighting, including those in the Kurdistan region - previously regarded as one of the safest parts of the country.
Earlier, David Cameron insisted the world must help the Yazidis in their "hour of desperate need" as he backed US president Barack Obama's decision to respond to a request by the Iraqi government for targeted air strikes.
The Prime Minister said he utterly condemned the "barbaric attacks" by IS - formerly known as Isis (the Islamic State of Iraq and the Levant).
"I am extremely concerned by the appalling situation in Iraq and the desperate situation facing hundreds of thousands of Iraqis. I am especially concerned for the minority Yazidi community now trapped on Mount Sinjar, where they have fled for their lives.
"They fear slaughter if they descend back down the slopes but face starvation and dehydration if they remain on the mountain. The world must help them in their hour of desperate need."
The US president announced in a late-night televised statement that military planes carried out humanitarian air drops in the region and said America would take action if the lives of its troops in Iraq are at risk from Islamic militants.
Archbishop of Canterbury Justin Welby said that religious minorities in the region - including Christians who have also been targeted by IS militants - were enduring "terrible suffering".
He said such attacks were part of an "evil pattern" around the world where minorities are being killed and persecuted for their faith.
"It is extremely important that aid efforts are supported and that those who have been displaced are able to find safety. I believe that, like France, the United Kingdom's doors should be open to refugees, as they have been throughout history," he said.
"The international community must document human rights abuses being committed in northern Iraq so that future prosecutions can take place. It is important and necessary for the international community to challenge the culture of impunity which has allowed these atrocities to take place.
"We must continue to cry to God for peace and justice and security throughout the world. Those suffering such appalling treatment in Iraq are especially in my prayers at this time."
The Roman Catholic Archbishop of Westminster, Vincent Nichols, joined the appeals for international help for those communities "facing a threat to their very existence in their biblical homelands".
"It is imperative that the international community ensure the physical protection of all communities in Iraq, their human rights including the right to religious freedom," he said.
"I urge Her Majesty's Government to lead the efforts in the face of such a human calamity in order to help restore these shattered communities, provide them with urgent humanitarian aid and work with others to ensure their long-term security in the land of their birth."
The Department for International Development later gave details of the UK's £8 million emergency aid package.
It includes £2 million of humanitarian supplies for 75,000 people, such as reusable filtration containers filled with clean water, tents, and solar lights which can also recharge mobile phones. Much of the aid can be dropped from the air to help those trapped in the Sinjar Mountains.
Some £3 million will go to charities and NGOs already on the ground and helping displaced people in northern Iraq, and £2.5 million to the International Committee of the Red Cross (ICRC).
A further £500,000 will be used to ensure Kurdish and UN systems can co-ordinate properly.
International Development Secretary Justine Greening said: "The world has been horrified by the brutal persecution of vulnerable minority groups by Isil extremists in Iraq. Hundreds of thousands of people have fled their homes and we are extremely concerned for their safety.
"This aid from the British people will help the Yazidi community, who are now cut off on Mount Sinjar, get immediate emergency support. It will also ensure thousands more people get medical help, shelter, food and clean water.
"It is absolutely vital that the UN gets the access it needs and the British government is working with the international community to push for this."