Pope Francis has issued a stinging rebuke of Myanmar over mass atrocities against Rohingya Muslims, including the slaughter of babies, in what the United Nations says "very likely" amount to crimes against humanity.

"They have been suffering, they are being tortured and killed, simply because they uphold their Muslim faith," Francis said in his weekly audience at the Vatican.

The rebuke comes as UN officials dealing with almost 70,000 Rohingya who have fled to Bangladesh refugee camps expressed concern the outside world has not fully grasped the severity of the crisis unfolding in Myanmar's western Rakhine State, home to almost one million Rohingya, who are denied basic rights, including citizenship in the country where they have lived for generations.

"The talk until now has been of hundreds of deaths. This is probably an underestimation – we could be looking at thousands," a UN official told Reuters.