Pope Francis said Monday that he wants to visit China and called for the Catholic Church in the country to be allowed to do its "job".

"You ask me if I want to go to China? Certainly, even tomorrow," he told reporters on board the papal plane as he returned from a visit to South Korea.

"But the church asks for the freedom to do its job in China, there is no other condition," he said.

During the five-day visit to the divided Korean peninsula -- his first to Asia -- the pope called for countries like China and Vietnam that do not have formal ties with the Vatican to accept a "dialogue" with Rome, insisting that Catholics did not view Asia with the mentality of "conquerors".

Speaking on Monday as the Korean Airlines plane he was travelling in flew over China, he said the Vatican is "always open to contact" and that he has a "high regard" for the Chinese people.

Pope calls for UN rather than US action in Iraq
Aboard The Papal Plane (AFP) Aug 18, 2014 - 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.

"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.