Pope Francis said on Monday the international community would be justified in stopping Islamist militants in Iraq, but that it should not be up to a single nation to decide how to intervene in the conflict, according to The Associated Press.

The leader of the world's 1.2 billion Roman Catholics made his comments in an hour-long conversation with reporters aboard a plane returning from a trip to South Korea that ranged from international diplomacy to his health and future travel plans, the AP reported.

During the encounter that has become a tradition at the end of his foreign journeys, Francis, 77, also said he planned to visit the United States next year and that he was ready to go to China "tomorrow" if the communist government allowed him, according to the AP.

Francis said he realised he had to slow down and be more "prudent" with his health and that he had learned how to handle the super-star status he has gained since coming to office last year by thinking of his errors and his own imminent mortality, the AP reported.

Francis was asked if he approved of U.S. strikes against Islamic State insurgents who have recently forced Christians and other minorities to flee their homes in Iraq, according to the AP. "In these cases, where there is an unjust aggression I can only say that it is legitimate to stop the unjust aggressor," he said.

The pope was careful not to give the impression that he was giving an automatic green light for military strikes, but he did not rule them out, the AP reported. He said the situation was grave and the international community had to respond together.

"I underscore the verb 'to stop'. I am not saying 'bomb' or 'make war', but stop him (the aggressor). The means by which he can be stopped must be evaluated. Stopping the unjust aggressor is legitimate," Francis said, according to the AP.

"One single nation cannot judge how he is to be stopped, how an unjust aggressor is to be stopped," Francis said, the AP reported. He said the United Nations was the proper forum to consider whether there was unjust aggression and how to stop it.