David Cameron joined with Barack Obama yesterday in committing the two countries to stopping "genocide" in Iraq, as the first consignment of UK emergency aid left RAF Brize Norton in Oxfordshire.
The Prime Minister and the US President discussed by telephone planned air drops of drinking water and other essential supplies to refugees around the Sinjar mountains, where thousands from the Yazidi minority religious group have been trapped for a week.
A Downing Street spokesman said: "The Prime Minister welcomed the US efforts and made clear that we are keen to work with the Americans on the humanitarian effort.
"They agreed that the immediate priority is to get vital supplies to those trapped on Mount Sinjar and the UK will join the US in delivering aid drops.
"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."
Britain has ruled out military action at this stage. Instead, the Government announced an £8m package of aid, including reusable filtration containers, tents and solar lights, which can also recharge mobile phones.
Some £3m will go to charities and NGOs already on the ground and helping displaced people in northern Iraq, and £2.5m will go to the International Committee of the Red Cross.
Supplies including drinking water and tents are due to be dropped from two RAF C130 aircraft, which flew out of Britain yesterday afternoon.
Speaking after chairing a meeting of the Government's Cobra committee, Foreign Secretary Philip Hammond said: "We can expect a continuing drumbeat of air-drop operations working in co-ordination with the US and, potentially, with others as well."
He added that any new Iraqi administration must be prepared to lead the fight against jihadists such as the militants of Isis.
"We are waiting, all of us are waiting, for a new Iraqi government to form which will then have to take the lead in responding to the challenge that Isis is posing to the integrity of the Iraqi state," Mr Hammond said.