Pope Francis on Thursday took wealthy nations to task over unequal healthcare and systems which increasingly penalise all but the very wealthy.

Healthcare is a particularly hot-button issue in the United States, where President Donald Trump has been fighting to scrap the Affordable Care Act brought in by his predecessor Barack Obama.

"Increasingly sophisticated and costly treatments are available to ever more limited and privileged segments of the population," the 80-year old pontiff said in an address read out to a World Medical Association conference.

The "growing divide ...raises questions about the sustainability of healthcare delivery," he said, denouncing "a systemic tendency toward growing inequality in healthcare".

Pointing the finger at "the richest countries, where access to care risks depending on financial means rather than needs", he said: "The state cannot renounce its duty to protect all those involved".

Francis's dressing down came just days before the Catholic Church celebrates its first-ever World Day for the Poor on Sunday.

As part of the lead-up, the Argentine made a surprise visit to a medical centre for the poor and homeless in St Peter's square Thursday, warmly clasping the hands of patients and volunteer staff alike at the Vatican's "field hospital".