Pope Francis on Wednesday called for all efforts to be made to avoid a "humanitarian catastrophe" in Gaza, and expressed concern at risk of a widening of the Hamas-Israel conflict.

"The victims are increasing and the situation in Gaza is desperate. Please do everything possible to avoid a humanitarian catastrophe," the 86-year-old said at the end of his weekly audience at the Vatican.

He did not explicitly mention the blast that tore through a hospital in Gaza late Tuesday killing hundreds of people, and for which Israel and Palestinians have traded blame.

"The possible widening of the conflict is worrying, while there are so many fronts already open across the world," the head of the worldwide Catholic Church added.

"Let the weapons be silent, let the cry for peace of the poor, of the people, of the children be heard.

"War does not resolve any problems, it only spreads death and destruction. It increases hatred and multiplies revenge. War erases the future."

Speaking in front of thousands of people in St Peter's Square, he added: "I urge believers to take only one side in this conflict — that of peace."

He called for a day of prayer, fasting, and penance on 27 October, including a gathering in the evening at St Peter's Square.

He invited members of different Christian denominations and other religions to join in.

Pope Francis last weekend called for humanitarian corridors to allow the delivery of essentials into Gaza, which is under heavy Israeli bombardment following a bloody attack by Hamas.