The Royal Air Force (RAF) will start dropping humanitarian aid supplies in northern Iraq "imminently", the foreign secretary has said.
Philip Hammond said the aid would help thousands of members of the Yazidi minority, who are trapped on a mountain after fleeing from Islamist militants.
Prime Minister David Cameron and US President Barack Obama discussed the situation over the phone earlier.
No 10 said they had agreed "a way must be found... to avert a genocide".
The Downing Street spokesman said estimates of the number of those trapped ranged from 50,000 to 150,000.
He said there was little information about what supplies they had with them.
"They face a desperate choice between risking dehydration and starvation or descending into the barbaric hands of... terrorists [from the group now calling itself the Islamic State]," he said.
The spokesman said Mr Cameron and Mr Obama had agreed the immediate priority was to get supplies to those trapped on Mount Sinjar, with the UK joining the US in delivering aid.
"Both leaders also agreed that aid drops are not a long-term solution, and that a way must be found to get these people to safety and to avert a genocide," the spokesman said.
"Discussions in the international community have yet to identify a safe passage for them and so officials have been tasked with working with the Kurds who have forces on the ground in the region, the United States, neighbouring countries and international aid organisations to explore how this could be done."
Mr Hammond earlier said the air drops, in co-ordination with the US, could be expected to continue for the foreseeable future.
Two cargo planes carrying aid left RAF Brize Norton on Saturday and have arrived in the region.
The C-130 planes were carrying supplies including reusable filtration containers filled with clean water, and solar lights that can recharge mobile phones as part of an £8m package of aid from the UK government.
Mr Hammond said: "We can expect a continuing drum beat of air drop operations, working in co-ordination with the US and potentially with others as well.
"More widely we are looking at how to support this group of [Yazidi] people and get them off that mountain, how we are going to facilitate their exit from what is a completely unacceptable situation.
"All of us are waiting for a new Iraqi government to be formed which will then have to take the lead in responding to the challenge that [militant group] Isis [now calling itself the Islamic State] is posing to the integrity of the Iraqi state," Mr Hammond said.
The Islamic State (IS) has recently made gains in northern Iraq and is threatening the capital of the Kurdish semi-autonomous region, Irbil.
Thousands of members of the minority Yazidis, who follow an ancient faith that IS condemns as devil worship, have fled into the mountains from the Islamist militants who are reported by Iraq's human rights ministry to have seized hundreds of Yazidi women.
Parallel to the aid effort, the US has carried out air strikes against IS targets.
Mr Cameron has welcomed the US strikes but previously ruled out the UK taking part in military action.
However, BBC political correspondent Alan Soady said the UK could support the US, by helping with air-to-air refuelling for planes or surveillance operations.
The UK's emergency aid package includes:
It brings the country's total support for the humanitarian effort in Iraq to £13m.
Meanwhile, the Foreign Office has advised Britons not to travel to those parts of Iraqi Kurdistan affected by fighting and to leave areas close to the conflict.