"It's immoral that the only thing we are doing is dropping food and water and leaving these people in the firing line of slaughter," said Conor Burns, a member of Parliament from Cameron's Conservative Party. "We're seeing beheadings, crucifixions, rapes - it's unbelievable that the West cannot get its moral compass together and go in and fight back."
Former British military leaders, meanwhile, said that airstrikes may not be enough and that Western powers may ultimately be forced to put boots on the ground.
"There need to be continuing airstrikes well targeted, and we may need to put some people on the ground to direct that quite accurately," Richard Dannatt, a former British army chief, told Radio 4's "Today" program on Monday. "It is difficult for us to say that this is not our problem. We have to look at ourselves and say, 'Do we do nothing in the face of a possible genocide?' "
President Barack Obama said over the weekend that the U.S. military intervention may go on "for months" but that he will not send in ground troops.
British officials signaled Monday that although a military response has not been ruled out, it's not on the table now, and that the government will focus on humanitarian assistance. That effort has consisted of a single successful airdrop of supplies to a group of Yazidis, an ethnic and religious minority, who have been trapped on northern Iraq's Mount Sinjar by advancing Islamic State fighters.
"We don't envisage a combat role at the present time," British Foreign Secretary Philip Hammond told the BBC. "We are talking at the moment about a humanitarian operation and using our assets to gain more awareness of what's going on on this mountain."
Cameron is in Portugal on summer break and is said to have no plans to return home early. Analysts say he is wary of an open-ended military engagement in Iraq, especially after a 2011 bombing campaign in Libya strained the resources of Britain's air force.
"If we get involved at the tip of an American iceberg, then we could end up making a larger commitment than we could comfortably make," said Michael Clarke, director general of the Royal United Services Institute, a London-based think tank.
Should Parliament be recalled this month to debate military intervention in Iraq, it would come just a year after Cameron suffered an embarrassing defeat in the House of Commons on a vote to authorize military force against the forces of Syrian President Bashar al-Assad. The vote helped to derail U.S. plans to bomb Syria over allegations of chemical-weapons use by the Assad government.
Cameron's critics suggested Monday that the memory of that loss may be preventing him from acting more forcefully in Iraq.
"The Prime Minister may feel unable to act now following his defeat and mishandling of the Syria debate last August," Mike Gapes, a Labor member of Parliament, wrote in a piece published in the New Statesman. "He should get over it and urgently recall Parliament."