David Cameron made the announcement hours after jetting back to the UK from a family holiday in Portugal.
The Prime Minister chaired a meeting of the government's emergency committee on the escalating crisis in northern Iraq,
He said the UK would be stepping up its role, taking part in a joint effort to evacuate the up to 30,000 people on Mount Sinjar, many of whom have no shelter or water. However, he insisted there were no plans for military intervention and that there was no need to recall MPs to vote on further action.
Earlier, the UK agreed to transport arms to Kurdish rebels fighting the group. It has also sent Tornado jets to help with the humanitarian aid effort. Earlier this week Number 10 suggested the UK could follow the Americans and consider arming the Kurds. Mr Cameron said that "detailed plans" were being put in place for Britain to help rescue stranded Yazidi refugees.
He said: "We need a plan to get these people off that mountain and get them to a place of safety. I can confirm that detailed plans are now being put in place and are under way and that Britain will play a role in delivering them.
While he would not be drawn on details of the mission, he said: "Britain will play a role with British planes and British aid helping to keep these people alive in a really desperate situation."
Earlier this week UN experts called on the international committee to do all it could to prevent a massacre in Iraq.
Mr Cameron has come under pressure from MPs in his party not to stand by while "ethnic cleansing" takes place.
Former Tory defence secretary Liam Fox has urged the UK to increase its involvement, warning of the dangers of "wishful thinking at best, and catastrophic complacency at worst" over the threats posed by the Islamic State group.
However, Mr Cameron said: "The first thing is to deal with this desperate humanitarian situation with people who are exposed, starving, dying of thirst on this mountain side, getting them to a place of safety."
He added on the issue of the Kurdish forces: "In terms of the ammunition they are getting, Britain is going to be playing a role in helping to get that to them."
The UK is expected to continue aid drops as plans to rescue the refugees are drawn up.
Last night Labour leader Ed Miliband said: "We must play our part in bringing help and relief to those thousands of refugees, and the UK must do all that it can to assist the vital international effort to bring them to safety."
As well as jets, helicopters are en route to Cyprus ready for possible deployment to Iraq.
France has announced it is meeting, with the agreement of the Iraqi government in Baghdad, a Kurdish call for arms to reinforce its peshmerga fighters' attempts to repel the IS advance.
Another 130 US troops have also arrived in Iraq on what the Pentagon described as a temporary mission to assess the scope of the humanitarian crisis.
Tory MP Mark Field, a member of the Commons Intelligence and Security Committee, called on Mr Cameron to level with the public about potential intervention.
He also said he expects "substantial numbers" of Christians and other minorities in Iraq to be offered asylum in Western countries including Britain.
EU foreign ministers will meet tomorrow to discuss Iraq and the latest developments in Ukraine.