Pope Francis on Sunday slammed the "immoral" use of force in Lebanon and Gaza, amid ongoing Israeli strikes in both places.

"A country that acts this way with force, no matter the country, and that acts in such an excessive manner, (lends itself to) immoral actions," said Francis when asked about the consequences of Israeli airstrikes on civilians aboard a flight back to Rome from Belgium.

"Defence must always be proportional to the attack. When this is not the case, a dominating tendency appears that goes beyond morality," the 87-year-old pontiff said in Italian.

"Even in war there is a morality to defend. War is immoral, but the rules of war indicate a form of morality," Francis said.