Pope Francis has issued his toughest condemnation yet of the invasion of Ukraine, saying the “unacceptable armed aggression” must stop.

Speaking to thousands of people in St Peter’s Square for his Sunday blessing, Francis also said the killing of children and unarmed civilians was “barbaric” and had “no valid strategic reason”.

He called besieged Mariupol a “martyred city” and again appealed for “truly secure humanitarian corridors” to allow residents to leave.

“In the name of God, I ask you: stop this massacre,” the pope said, adding that Ukrainian cities risked “being reduced to cemeteries”.

The pope has not used the word “Russia” in his condemnations of the war since Vladimir Putin ordered the invasion on 24 February. However, his choice of words, such as “armed aggression” and “no valid strategic reason”, appeared to be aimed at contesting Moscow’s justifications for the invasion.

Russia calls its action a “special military operation”. Last Sunday Francis implicitly rejected that term, saying it could not be considered “just a military operation” but a war that had unleashed “rivers of blood and tears”.

Moscow says its action is designed not to occupy territory but to demilitarise and “denazify” its neighbour. It has denied targeting civilian areas.