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 Islamic State 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?"

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