The international community is justified in stopping Islamist militants in Iraq, Pope Francis has said.

However, he added 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.

He was aboard a plane returning from a trip to South Korea that ranged from international diplomacy to his health and future travel plans.        

Pope Francis was asked if he approved of US strikes against Islamic State 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.         

Meanwhile, US President Barack Obama has said that the United States has embarked on a long-term mission to defeat the insurgents of the so-called "Islamic State" fighting in Iraq.

Ten days after ordering air strikes against the jihadist fighters, Obama warned that IS remains a threat to Iraq and the wider region, telling Baghdad "the wolf is at the door."