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Pope Francis said on Monday that those killed in conflicts such as Gaza and Ukraine should not be considered "collateral damage", as he condemned the "war crimes" perpetrated against civilians.
"The distinction between military and civil objectives is no longer respected," the Pope, 87, said in his new year's address to diplomats at the Vatican.
"There is no conflict that does not end up in some way indiscriminately striking the civilian population. The events in Ukraine and Gaza are clear proof of this.
"We must not forget that grave violations of international humanitarian law are war crimes."
Pope Francis's speech was dominated by calls for an end to conflicts around the world.
He said people "need to realise more clearly that civilian victims are not 'collateral damage' but men and woman, with names and surnames, who lose their lives".
"Were we to be able to look each of them in the eye, call them by name, and learn something of their personal history, we would see war for what it is – nothing other than an immense tragedy," the Pope said.
The pontiff again condemned the October 7 attack by Hamas militants, which resulted in about 1,140 deaths in Israel, mostly civilians, according to an AFP tally based on official Israeli figures.
The Palestinian group, considered a "terrorist" organisation by the US and EU, also took about 250 hostages, 132 of whom remain captive, Israel says. At least 24 are believed to have been killed.
"I renew my condemnation of this act and of every instance of terrorism and extremism," the Pope said.
Latest from the Israel-Gaza war - in pictures
Israel has responded since October 7 with relentless bombardment of Gaza and a ground invasion that have killed at least 22,835 people, most of them women and children, according to the Gaza Health Ministry.
The pontiff said Israel's military response "has caused an exceptionally grave humanitarian crisis and inconceivable suffering".
"To all the parties involved, I renew my appeal for a ceasefire on every front … and the immediate liberation of all the hostages held in Gaza," Pope Francis said.
"I ask that the Palestinian people receive humanitarian aid, and that hospitals, schools and places of worship receive all necessary protection."
Most of Gaza's population has been displaced, according to the UN, leaving them in overcrowded shelters or tents in the winter cold.
The World Health Organisation has warned of the risk of famine and disease, with people struggling to find food, safe water, fuel and medicine.
On Sunday, the UN reported "sickening scenes" at Al Aqsa Martyrs' Hospital, one of the few in Gaza that is still partly functioning, with "people of all ages being treated on blood-streaked floors and in chaotic corridors".
