The Pope says ISIS must be stopped. But how?

By Daniel Burke, CNN Belief Blog Editor

[twitter-follow screen_name='BurkeCNN']

(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.

During the wide-ranging papal press conference, Francis said he would like to visit Philadelphia in 2015, and perhaps New York and Washington as well, where he's been invited to address the UN and U.S. Congress.

The pontiff also extended an olive branch to China, waved off concerns about his health, and said he tries to live a normal life, despite his enormous popularity and job pressures.

But Francis' comments on Iraq are likely to draw the most debate.

On Monday, President Barack Obama said that the "targeted airstrikes" in Iraq that began on August 8 have effectively protected U.S. personnel, helped secure the Kurdish city of Irbil and allowed humanitarian aid to reach persecuted religious minorities.

U.S. fighter jets and drones have bombed Sunni Islamic extremists in northern Iraq, aiming at ISIS artillery units and convoys, according to Obama administration officials.

U.S. warplanes, Kurdish forces pound ISIS targets

ISIS has been seizing control of towns and key infrastructure in an advance that has forced hundreds of thousands to run for their lives. The Sunni militants have also executed people who don't share their puritanical interpretation of Islam, posting videos of their killings to the Internet.

In his press conference on Monday, Pope Francis made clear that his concerns for Iraqis extends beyond Christians.

"The martyrs, there are many martyrs," he said. "But here there are men and women, religious minorities, not all of them Christian, and they are all equal before God."

The American airstrikes have created a sharp moral dilemma for some Christian leaders, such as the Pope, who abhor violence and war in general but have repeatedly called for the international community to help the thousands of Yazidis, Christians, Muslims and other religious minorities living in fear of ISIS.

The Catholic Church has a robust tradition of sanctioning "just wars." But too often, the Pope said Monday, such wars beget an immoral mission creep and are later used to justify wars of conquest.

Last September, Francis called for a day of fasting to persuade the United States and other nations not to use force in Syria. "Never has the use of violence brought peace in its wake," he said at the time. "War begets war, violence begets violence."

In Iraq, however, military action may now be necessary, Archbishop Silvano Tomasi, the Vatican's UN ambassador, said last week.

(Chelsea J. Carter, Tom Cohen and Barbara Starr contributed to this report.)