British planes are to drop emergency aid supplies for Iraqi refugees living under threat from militants.
Members of the Yazidi community trapped on a mountainside will be among those to receive water, tents and lights.
The drops are part of an £8m package of aid announced by the UK government, bringing its total support to £13m.
Prime Minister David Cameron has welcomed US air strikes against Islamic State (IS) militants but has ruled out any UK military action.
The IS militants - a Sunni Muslim group formerly known as Isis or Isil (Islamic State in Iraq and the Levant) - have seized Qaraqosh, Iraq's biggest Christian town, prompting members of the Yazidi religious community to flee.
Officials have warned the Yazidis face starvation and dehydration if they remain stranded on Mount Sinjar, and slaughter at the hands of the militants if they move.
US troops have already made humanitarian air drops to Iraqis threatened by IS.
International Development Secretary Justine Greening said the world had been "horrified by the brutal persecution of vulnerable minority groups" in Iraq.
"Hundreds of thousands of people have fled their homes and we are extremely concerned for their safety," she said.
"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."
She added it was "absolutely vital" that the UN secured access to those in need.
Sir Jeremy Greenstock, the UK's former ambassador to the United Nations and special representative to Iraq, told BBC Radio 4's Today programme he was "quite surprised" more had not already been done.
"Let's get on with it - these people are in desperate trouble", he added.
Sir Jeremy said he did not expect the UK to join the US in military action.
He said: "I expect the government to be concentrating on humanitarian relief and not get involved in offensive air strikes."
The new emergency aid package includes:
- £2m of emergency supplies for 75,000, including aid to be dropped to those trapped on Mount Sinjar
- The supplies will include resusable filtration containers filled with clean water, solar lights that can also be used to recharge mobile phones, and tents and tarpaulins
- £3m of fast-tracked funding for charities and NGOs currently operating in Iraq
- £2.5m of support for the International Committee of the Red Cross (ICRC)
- £500,000 to help coordinate the relief efforts
The prime minister said the world must help religious minorities in Iraq who were under threat from the militants "in their hour of desperate need".
BBC political correspondent Iain Watson said that while Downing Street could not rule out military action indefinitely, the mood was not "gung-ho" ahead of the 2015 general election.
Meanwhile, the Foreign and Commonwealth Office has advised Britons not to travel to those parts of Iraqi Kurdistan affected by fighting and to leave areas close to the conflict.
After chairing a meeting of the government's emergency Cobra committee on the situation in Iraq, Defence Secretary Michael Fallon 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."
Asked why the UK was not intervening militarily, he added: "We welcome what America is doing in particular to bring humanitarian relief and to prevent any further suffering - but our focus is on assisting that humanitarian mission, using our military in support of the Americans in terms of refuelling and surveillance."
Archbishop of Canterbury Justin Welby has urged the UK to welcome Christians and other religious minorities fleeing Iraq.
He said: "What we are seeing in Iraq violates brutally people's right to freedom of religion and belief, as set out under Article 18 of the Universal Declaration of Human Rights.
"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."
'Right to freedom'
In an earlier statement, Mr Cameron said: "I welcome President Obama's decision to accept the Iraqi government's request for help and to conduct targeted US air strikes, if necessary, to help Iraqi forces as they fight back against Isil terrorists to free the civilians trapped on Mount Sinjar.
"And I fully agree with the president that we should stand up for the values we believe in - the right to freedom and dignity, whatever your religious beliefs."
A Downing Street spokeswoman said the UK was "not planning a military intervention".
By Jonathan Marcus
The UK is especially concerned at the developing humanitarian crisis in Iraq as religious minorities flee the advance of Islamic State forces.
For now British military help may simply not be needed. It is the Americans who have been asked by the Iraqi government to intervene and Washington has more than enough capacity in the region if air strikes are required.
Britain, along with the Americans and others, is encouraging the Iraqi Prime Minister Nouri al Maliki to pursue more inclusive governing arrangements.
His behaviour is seen as partly responsible for many of Iraq's current problems, and its difficulties in confronting the Islamic State challenge.
Labour's shadow foreign secretary Douglas Alexander said President Obama was right to authorise limited air strikes.
"The British government should be speaking up, speaking out, and acting quickly to ensure that the international community does not simply walk by on the other side as this persecution increases," he said.
But he added that international action alone - though "vital to help prevent a brutal and horrific escalation" - would not solve the crisis in the long term and it would be up to Iraqis themselves to achieve political reconciliation and a strengthening of their security forces.