Pope Francis has suggested that he may not have long left as the head of the Catholic Church in a revealing interview given yesterday during his plane trip back from Korea.

In response to a question about his unprecedented popularity, the Pontiff responded: "I see it as the generosity of the people of God.

"I try to think of my sins, my mistakes, not to become proud. Because I know it will last only a short time. Two or three years and then I'll be off to the Father's House," he quipped.

Francis' predecessor, Pope Benedict XVI, was the first to retire from papacy in almost 600 years – the last being Pope Gregory XII in 1415 – and Francis himself has hinted that he himself may well follow suit, "even if it does not appeal to some theologians".

"Benedict XVI opened a door," he added, noting that even Catholic bishops rarely retired several decades ago, but now the practice is common.

Francis' words echo his comments earlier this year, during an interview in which he said he wouldn't "rule out" retirement.

"I will do what the Lord tells me to do," he told reporters on another plane journey, this time back from the Middle East.

The Pope also spoke out about the escalating crisis in Iraq yesterday, and stressed the importance of intervention against Islamic State (IS) insurgents.

Asked whether he thought US military strikes were justifiable, Francis replied: "In these cases, where there is an unjust aggression I can only say that it is legitimate to stop the unjust aggressor.

"I underscore the verb 'to stop,'" he continued. "I am not saying 'bomb' or 'make war', but stop him. The means by which he can be stopped must be evaluated. Stopping the unjust aggressor is legitimate."

Francis also highlighted the role of the UN in situations of crisis, insisting: "One single nation cannot judge how he is to be stopped, how an unjust aggressor is to be stopped."

Having sent his personal envoy, Cardinal Fernando Filoni, to northern Iraq last week, the Pope even shared that he had considered travelling to the fractured region himself, but "At this moment, it would not be the best thing to do.

"But I am willing to do it," he said.

On Sunday, the Bishop of Leeds criticised the British government's approach to Islamic extremism across the globe, including Iraq.

"We do not seem to have a coherent or comprehensive approach to Islamist extremism as it is developing across the globe," Bishop Nick Baines wrote in a letter to PM David Cameron, published in the Observer.

"Does your government have a coherent response to the plight of these huge numbers of Christians whose plight appears to be less regarded than that of others? Or are we simply reacting to the loudest media voice at any particular time?"

On Monday, a video released by IS warned the US that it will attack Americans "in any place" if air strikes successfully target its fighters, Reuters reports.

It also contained the statement: "We will drown all of you in blood."