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.

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.

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.

He 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.

Francis was asked if he approved of US strikes against Isis insurgents who have recently forced Christians and other minorities to flee their homes in Iraq.

"In these cases, where there is an unjust aggression I can only say that it is legitimate to stop the unjust aggressor," he said.

Proclaiming a caliphate straddling Iraq and Syria, the militants have swept across northern Iraq, pushing back Kurdish regional forces and driving tens of thousands of Christians and members of the Yazidi religious minority from their homes.

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. 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," he said.

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

The pope disclosed that he had considered going to Iraq after his return from Korea, but decided against a visit for the time being. "At this moment, it would not be the best thing to do, but I am willing to do it," he said.

He has sent a senior cardinal to Iraq to visit refugees and distribute Vatican charity funds and sent a letter to UN secretary general Ban Ki-moon about the need to stop the bloodshed.

Francis said he wanted to go Philadelphia in September 2015 for a meeting of Catholic families and hinted that the trip might well include a visit to the White House and Congress (in Washington DC) and the UN (in New York).

That trip, which would be his first to the US, could also be expanded to include Mexico, he said, but no decision had been taken.

During his five-day visit to South Korea, Francis sent several signals to China, which does not allow Catholics to recognise the pope's authority, saying the communist government there should not fear Christians because they did not want to "come as conquerors" but be integral parts of local cultures.

On the plane, he said the Vatican was always open to dialogue with Beijing, calling the country "noble and wise", but said that the church needed to carry out its mission in freedom.

Poking fun at himself several times, Francis said that his custom of not taking vacations outside the Vatican was one of his "neuroses". He said he had slowed down for the summer by reading more, sleeping more, and listening to music.

"Now I have to be more prudent, you are right," he told a reporter, who reminded him that he had been forced to cancel several events at the last minute in the past few month because of minor ailments or illnesses.

Francis said he did not let fame go to his head by thinking of his "sins and mistakes" and remembering that "this will last a short time, two or three years, and then we go to the house of the father".

The pope, who stood for the entire hour-long conversation, then made a chopping gesture with his hand and a whistling sound as if to say death comes sooner or later for everyone.

The pope also opened the way to a quick beatification for Oscar Romero, saying there are no more doctrinal problems blocking the process for the murdered Salvadoran archbishop who is one of the heroes of the liberation theology movement in Latin America.

Romero, the archbishop of San Salvador, was gunned down in 1980 while celebrating mass. He had spoken out against repression by the Salvadoran army at the beginning of the country's 1980-1992 civil war between the right-wing government and leftist rebels.

Francis told journalists travelling home from South Korea that Romero's case had previously been "blocked out of prudence" by the Vatican's Congregation for the Doctrine of the Faith but has now been "unblocked." He said the case had passed to the Vatican's saint-making office.

The congregation launched a crackdown on liberation theology under then-Cardinal Joseph Ratzinger, fearing what was deemed as Marxists excesses. The movement holds the view that Jesus' teachings imbue followers with a duty to fight for social and economic justice.

Francis said of Romero's case that "it is important to do it quickly," but that the investigation must take its course.

He declared that Romero "was a man of God" and suggested that he wanted to expand the church's concept of martyrdom to include a broader field of candidates.

Unlike regular candidates for beatification, martyrs can reach the first step to possible sainthood without a miracle attributed to their intercession. A miracle is needed for canonisation, however.

Traditionally, the church has restricted the martyr designation to people who were killed out of hatred for the Catholic faith. Francis said he wanted theologians to study whether those who were killed because of their actions doing God's work could also be considered martyrs.

"What I would like is that they clarify when there's a martyrdom for hatred of the faith for confessing the faith as well as for doing the work for the other that Jesus commands," Francis said.

Questions over that distinction have been at the root of the theological debate over whether Romero was killed by El Salvador's right-wing death squads for professing the faith or because of his political activism in support of the poor.