David Cameron has returned to Downing Street from his holiday in Portugal a day earlier than expected to assume direct command of the government's response to the Iraq crisis.

A tweet from the prime minister's official Twitter account announced that Cameron had returned to No 10 to allow him to chair a meeting of the government's emergency Cobra meeting on Thursday. It said: "The PM is now back in the UK and will chair the routine daily COBR at 13.00 for latest updates on situation in Iraq."

The prime minister had been expected to return to Downing Street on Thursday. His decision to return home a day earlier than expected followed criticism from Tory MPs of the prime minister for allowing himself to be photographed on a Portuguese beach while RAF crews were flying dangerous missions to drop humanitarian supplies to Yazidi refugees trapped on Mount Sinjar.

Backbench Tory critics were distributing a cartoon from Tuesday's Times that featured lines of dead refugees lying on the ground. The cartoon ended with a pot-bellied Cameron lying on a beach towel in relaxed pose.

The prime minister will chair the Cobra meeting as the government comes under renewed pressure to allow the British military to protect Iraqi civilians after the intervention of a former defence secretary and former military figures.

As more aid was delivered to help thousands of people trapped on Mount Sinjar in the north, Liam Fox attacked the "catastrophic complacency" of world leaders. His words came as the international development secretary, Justine Greening, confirmed on Tuesday that a third round of successful UK airdrops had taken place.

The supplies included two RAF C-130 consignments containing 2,640 reusable water purification containers filled with clean water. More than 500 shelter kits to provide shade in temperatures higher than 40C (104F) were also inside the packages. There have now been five successful drops over three nights.

Meanwhile, 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 faced by thousands of displaced civilians trapped on Sinjar.

Fox wrote in the Daily Mail: "The idea that this is not our problem is wishful thinking at best, and catastrophic complacency at worst. The US government has made a belated, but welcome, decision to use American air power to hit Isis bases. We should be willing to do the same if asked.

"Sending humanitarian aid is right but if we are leaving the vulnerable unprotected from the military terror of the Isis forces then our help is superficial."

Cameron has insisted the UK will not intervene militarily, despite increasing calls from experienced commanders for it to join the US in air strikes on Isis targets.

Greening, who announced £8m in new assistance last week: said: "As thousands of Iraqi people remain cut off away from their homes, we are focused on getting help to those in need, particularly those trapped on Mount Sinjar.

"The RAF have successfully made five drops, including thousands of containers filled with clean water that can also be used to purify dirty water and hundreds of shelter kits."

Col Tim Collins, noted for the 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 the UK joined military action, and dismissed the airdrops as a "pebble in the ocean". He also urged the government to arm and train Kurdish peshmerga fighters.

Gen Sir Mike Jackson, head of the army during the Iraq war, said there was a moral duty to help with the humanitarian response and joined calls for Britain to take part in air strikes. He told the newspaper he would have "no difficulty at all" in saying the UK should stand alongside the US to "up the British ante" in the use of air power on humanitarian grounds.

Labour welcomed the stepping up of aid efforts, including the deployment of Chinook helicopters, but did not join the calls for the UK to take part in the direct US military intervention. The 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.

"Clearly the UK must now also be doing all that we can to assist the vital humanitarian effort that is now under way to help save lives and prevent further suffering."