Aboard the papal plane (AFP) - Pope Francis called Monday for collective action through the United Nations to "stop unjust aggression" in Iraq, in an implicit criticism of unilateral US air strikes there.

The pope, speaking to reporters aboard his flight back from a trip to South Korea, said he was ready to visit Iraq "if necessary", providing it would help people under threat there.

Asked about the recent US strikes against Islamic State (IS) targets in the north of the country, Francis said that "in the case where there is unjust aggression, it is acceptable to stop an unjust aggressor. I emphasise the word 'stop'. I am not saying 'bombard' or 'make war upon'."

"One nation cannot decide" alone how to end the aggression, he said.

"The idea of the United Nations came after World War II. That is where we should be having the discussion and saying, 'There is an unjust aggressor. How are we going to stop it?'"

Last week as thousands from Iraq's Yazidi and Christian minorities fled attacks by IS jihadists, Francis made a plea to the UN to do all it could to stop the violence.

The Vatican's ambassador to the United Nations, Silvano Tomasi, had voiced support days earlier for the US air strikes, in a rare exception to papal policy promoting peaceful conflict resolution.

"Military action might be necessary," Tomasi had said.

The United States has launched nearly 70 air strikes in Iraq since early August to support Iraqi and Kurdish forces pushing back IS militants in the north.

The pope told reporters on Monday he wanted to clarify his position on the right to a legitimate defence in the face of genocide.

Church figures on the ground in Iraq have warned that persecution of Christian by militants could become a genocide.

Hundreds of thousands of people including many of Iraq's Christian minority have fled their homes in the north due to the rapid advance of the jihadists.

Francis said he and his staff were considering different options to help those threatened by the jihadists, included a potential trip to Iraq.

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"We said, if necessary, when we return from Korea we can go there, but right now it is not the best thing to do," he told reporters.

"I am available, and I am ready," the 77-year-old added.

The pontiff was returning from a five-day stay in South Korea, the first papal visit to Asia in 15 years.