Pope Francis has kicked off global Christmas celebrations with a call for peace, as Israel's war on Hamas and Russia's invasion of Ukraine cast a shadow over one of the world's favourite holiday.

Having said earlier in the day that he was thinking of people "who are suffering from war -- we are thinking of Palestine, of Israel, of Ukraine", the pope struck a sombre tone during his Christmas Eve mass.

"Tonight, our hearts are in Bethlehem, where the Prince of Peace is once more rejected by the futile logic of war, by the clash of arms that even today prevents him from finding room in the world," the pope said.

The biblical city in the occupied West Bank, where Christians believe Jesus Christ was born in a stable more than 2,000 years ago, effectively cancelled the annual Christmas celebrations that normally draw thousands of tourists.

The town did away with its giant Christmas tree, marching bands and flamboyant nativity scene this year, settling for just a few festive lights.

In the centre of town, a huge Palestinian flag had been unfolded with a banner declaring that "The bells of Bethlehem ring for a ceasefire in Gaza".

"A lot of people are dying for this land," said Nicole Najjar, an 18-year-old student.

"It's really hard to celebrate while our people are dying."

The Latin patriarch of Jerusalem, Pierbattista Pizzaballa, said: "We are here to pray and to ask not only for a ceasefire, a ceasefire is not enough, we have to stop these hostilities and to turn the page because violence generates only violence".

Sister Nabila Salah from the Catholic Holy Church in Gaza -- where two Christian women were killed by an Israeli sniper earlier this month, according to the Latin Patriarchate of Jerusalem -- told AFP "all Christmas celebrations have been cancelled".

"How do we celebrate when we are... hearing the sound of tanks and bombardment instead of the ringing of bells?" she said.

In Syria, churches limited celebrations to prayers in solidarity with the Palestinians.