Francis also said he and his advisers were considering whether he might go to northern Iraq himself to show solidarity with persecuted Christians. But he said he was holding off for now on a decision.
In other comments to journalists returning from South Korea, Francis confirmed he hoped to travel to the United States in September 2015 for a possible three-city tour: to attend a family rally in Philadelphia and to address Congress in Washington and the United Nations in New York.
On Iraq, Francis was asked if he approved of the unilateral US airstrikes on militants of the Islamic State who have captured swaths of northern and western Iraq and northeastern Syria and have forced minority Christians and others to either convert to Islam or flee their homes.
"In these cases, where there is an unjust aggression, I can only say that it is licit to stop the unjust aggressor," Francis said.
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"I underscore 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."
"One nation alone cannot judge how you stop this, how you stop an unjust aggressor," he said, apparently referring to the United States.
"After World War II, the idea of the United Nations came about: It's there that you must discuss 'Is there an unjust aggression? It seems so. How should we stop it?' Just this. Nothing more."
His comments were significant because the Vatican has vehemently opposed any military intervention in recent years, with St John Paul II actively trying to head off the Iraq war and Francis himself staging a global prayer and fast for peace when the US was threatening airstrikes on Syria last year.