David Cameron has returned from his holiday early in order to chair a meeting of the government's Cobra emergency committee on the crisis in Iraq.
The prime minister, who had not been due to resume charge until tomorrow, is under pressure to consider direct UK military intervention.
MPs from all parties, including a substantial number of Tories, have been also urged him recall parliament from recess to allow the Commons to debate Britain's involvement in any escalation of Western intervention. So far Downing Street has rebuffed the demands.
Conservative Conor Burns said: "Now that the PM is back I hope he will listen to those of us who want more action on Iraq and will allow that to be expressed in Parliament."
RAF aircraft have made several drops of humanitarian supplies to civilians trapped on a mountainside by advancing Islamic State (IS) extremists.
Chinook helicopters are to be sent to the region to increase the options to ease the plight of the tens of thousands of trapped Yazidis, and Britain is also to transport military equipment to resupply Kurdish forces which have been outgunned by IS.
Cameron - who flew back to the UK from Portugal on a scheduled fight ahead of his wife and children to chair the meeting - is expected to discuss progress on plans for a co-ordinated international rescue effort to extract the trapped Yazidis.
Another 130 US troops have arrived in Iraq on what the Pentagon described as a temporary mission to assess the scope of the humanitarian crisis.
But the prime minister has so far insisted the UK will not intervene militarily - despite increasing calls from experienced commanders and several Tory MPs.
A third round of successful UK air drops took place last night, including 2,640 reusable water purification containers filled with clean water. More than 500 shelter kits to provide shade in temperatures of more than 40C (104F) were also inside the packages. There have now been five successful drops over three nights.
Among those joining the call for firmer action was Tory former defence secretary Liam Fox. Writing in the Daily Mail today, he backed the use of the military to protect civilians from the "barbaric" jihadists and attacked what he called the "catastrophic complacency" of Western politicians over the humanitarian crisis.
He urged Cameron to send in RAF jets to join the US strikes and agree to supply arms to the Kurdish army.
Colonel Tim Collins, noted for the inspirational speech he delivered to troops on the eve of the 2003 Iraq War, warned in the Daily Telegraph that ancient civilisations in Iraq could be "extinguished" unless there is action and dismissed the aid drops as a "pebble in the ocean".
General Sir Mike Jackson, professional head of the Army during the Iraq War, said there was a "moral duty" to help with the humanitarian response.
He told the newspaper he would have "no difficulty at all" in saying the UK should be alongside the US to up the "British ante" to the use of airpower on humanitarian grounds.
Labour welcomed the stepping up of aid efforts, including the deployment of Chinook helicopters, but did not join the calls for direct UK military intervention.
Shadow foreign secretary Douglas Alexander said: "The Iraqi government has asked for international help. The UN Security Council has urged the world to act, and the United States has unique military capabilities to help avert a massacre.
"The Government should be working actively with our allies to now step up the humanitarian response to the unfolding tragedy on Mount Sinjar. That should include looking at how the United Kingdom can contribute most effectively."
Labour MP Graham Allen hit out at those calling for UK air strikes. "Many MPs who voted for the Iraq war, started this blood-letting and the creation of Isis, have learnt nothing and bay for yet more violence," he said.