The Pontiff revealed that he and his advisors were even considering whether or not he should go to northern Iraq to show solidarity with the persecuted Christians but stressed no decision had yet been made.
Returning from a visit to South Korea, he said: "In these cases, where there is an unjust aggression, I can only say it is legitimate to stop the unjust aggressor. I emphasise the verb 'stop'; I'm not saying 'bomb' or 'make war,' just 'stop'. And the means that can be used to stop them must be evaluated."
He stressed how throughout history such "excuses" to stop an unjust aggression had been used by world powers to justify a "war of conquest" in which an entire people were taken over.
"One nation alone cannot judge how you stop this, how you stop an unjust aggressor," declared Francis in an apparent reference to the US. "After World War II, the idea of the United Nations came about; it's there you must discuss - is there an unjust aggression? It seems so. How should we stop it?' Just this; nothing more."
His comments are significant given the Vatican has strongly opposed any military intervention in recent years. John Paul II actively tried to stop the Iraq war and Francis himself staged a global prayer and fast for peace when America threatened airstrikes against Syria last year.
Yet the Vatican has been increasingly showing support for military intervention in Iraq as Christians are being directly targeted because of their faith and their communities, which have existed for 2000 years, have been eradicated as a result of the extremists' onslaught.
Proclaiming a caliphate straddling parts of Iraq and Syria, the militants from the self-proclaimed Islamic State(IS) have swept across northern Iraq and driven tens of thousands of Christians and members of the Yazidi religious minority from their homes.
In London, the Prime Minister made clear Britain would not be "dragged into a war in Iraq". He was responding to fears of mission creep raised after Michael Fallon, the Defence Secretary, said the UK's involvement could last for months as it moved from simply humanitarian aid to helping with military planning.
Mr Cameron said the Coalition had a "fully worked through" strategy to tackle IS extremists and argued limited action was needed to prevent violence spreading to British streets.
He explained how the UK was ready to provide arms to Kurdish fighters against the "murderous extremists" in northern Iraq.
The PM, who cut short a break in Portugal earlier this month to respond to the emergency, is expected to head to Cornwall for another holiday this week but insisted he would be able to manage Whitehall's response to the crisis.
Mr Cameron maintained his resistance to recalling Parliament, saying it was unnecessary as "we are not contemplating things that would require that".
Labour and senior Church of England bishops complain the UK Government has no "coherent or comprehensive approach" to Islamist extremism and is failing to protect Christians.
But the PM hit back, saying: "We do want to have, and we do have, a fully worked through strategy for helping allies to deal with this monstrous organisation, IS."