Pope Francis on Saturday criticised the naming of the US military's biggest non-nuclear explosive as "the Mother of All Bombs", saying the word "mother" should not be used in reference to a deadly weapon.

The US Air Force dropped such a bomb, officially designated as the GBU-43 Massive Ordnance Air Blast (MOAB) on suspected Daesh fighters in eastern Afghanistan last month. The nickname was widely used in briefings and reporting on the attack.

"I was ashamed when I heard the name," Pope Francis told an audience of students on Saturday. "A mother gives life and this one gives death, and we call this device a mother. What is happening?"

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 Pope Francis is set to meet the US President Donald Trump on May 24 in a potentially awkward encounter given their opposing positions on immigration, refugees and climate change.