By Daniel Burke, CNN Belief Blog Editor

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(CNN) - Nearly everyone agrees that the militant Muslim group rampaging through northern Iraq must be stopped. The question is, how?

Asked if he approved of the American airstrikes against ISIS, Pope Francis withheld his moral imprimatur on Monday, refusing to fully support or denounce the military campaign.

"I can only say this: It is licit to stop the unjust aggressor," the pontiff said during a press conference on the plane back to Rome from South Korea.  "I underline the verb: stop. I do not say bomb, make war, I say stop by some means."

In an apparent reference to the United States, Francis said "one nation alone cannot judge" the best means of stopping groups like ISIS, which calls itself the Islamic State.

Those decisions should be made collectively by the United Nations, the pontiff said.

"It is there that this should be discussed. Is there an unjust aggressor? It would seem there is. How do we stop him?" the Pope asked, without answering his own question.

Already, Francis' cautious comments about American airstrikes and the use of force have fostered a welter of interpretations, from "tacit approval" to a "yellow light" to outright endorsement.

The Pope who was returning to Rome after a five day trip to South Korea, may soon have the chance to clarify his moral argument personally to U.S. and UN officials.

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