Pope Francis issued thinly veiled threats to Donald Trump on Monday afternoon as an ongoing feud between the pair continued.

Asked about the president's plans to rescind DACA, Francis said those who support the separation of families through immigration laws are not 'pro-life'.

Speaking to reporters aboard the papal plane returning from a five-day trip to Colombia, he said he hoped Trump 'rethinks' the decision to scrap the plan that allowed immigrants bought into the US as children to remain.

'I believe that this law comes, I think, not from Congress but from the executive branch of the government,' he said. 'If it is so, I have the hope that it's re-thought, because I heard the president of the United States introduce himself as a "pro-life" man.'

He added: 'A good pro-lifer understands that family is the cradle of life, and that its unity must be defended.'

Pope Francis also turned his ire to climate change deniers as Irma, the third hurricane in recent days, raged over Florida, disrupting the papal plane's return flight to Rome.

'Those who deny climate change need to go to scientists and ask them,' Francis said. 'They are clear and precise.'

Trump has repeatedly denied his belief in man-made climate change. 

Francis added that climate change is a 'serious matter over which we cannot make jokes' as he insisted scientists had been perfectly clear.

'Each [person] has a moral responsibility, bigger or smaller,' he said.

'History will judge the decisions.'

The pope was coming back from a 'peace and reconciliation' trip to Colombia where a violent civil war has caused havoc for more than 50 years.

'If Colombia wants a stable and lasting peace, it must urgently take a step in this direction, which is that of the common good, of equity, of justice, of respect for human nature and its demands,' he told thousands in the port city of Cartagena on Sunday.

'Only if we help to untie the knots of violence, will we unravel the complex threads of disagreements,' he said.