The British air force will drop food aid to Iraqi refugees fleeing extremists in "the next couple of days", the defence secretary said on Friday, although London has ruled out taking military action with the United States.
The announcement came after the Foreign Office urged Britons in the Arbil, Sulaimaniyah and Dohuk provinces of Iraq's autonomous Kurdistan region to "leave now" as fighting spreads north.
Prime Minister David Cameron said he was "extremely concerned by the appalling situation in Iraq and the desperate situation facing hundreds of thousands of Iraqis".
"And I utterly condemn the barbaric attacks being waged by ISIL (now Islamic State) terrorists across the region," he said in a statement.
The government's emergency committee, COBRA, met on Friday morning and agreed to help US humanitarian operations in northern Iraq and send the Royal Air Force (RAF) to drop food for stranded civilians.
The drops will be targeted at members of the minority Yazidi community who have fled from the extremists to the Sinjar mountains in northern Iraq.
"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," said Defence Secretary Michael Fallon.
"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."
A ministry of defence source confirmed this would involve RAF planes.
US President Barack Obama on Thursday authorised US warplanes to drop food and water to refugees and also agreed to targeted airstrikes, which began Friday.
A spokeswoman for Cameron's Downing Street office said however that Britain, which joined the US-led invasion of Iraq in 2003, was "not planning a military intervention".
In a statement, Cameron said: "I welcome President Obama's decision to accept the Iraqi government's request for help and to conduct targeted US airstrikes, if necessary."
And he urged international help for the Yazidis.
"They fear slaughter if they descend back down the slopes but face starvation and dehydration if they remain on the mountain," the prime minister said.
"The world must help them in their hour of desperate need."
In updated travel advice, issued before the US attacks began, the Foreign Office advised "against all travel to those areas of the Kurdistan region -- Arbil, Sulaimaniyah and Dohuk provinces -- affected by recent fighting".
"This follows attacks by the Islamic State of Iraq and the Levant (ISIL) on towns to the south west of Arbil on 6-7 August. If you're currently in these areas you should leave now," it said.