Pope uses word 'genocide' to describe mass murder of Armenians
A Sri Lankan policeman stands guard as front of the billboard bearing the image of Pope Francis in Wattala on January 12, 2015 ahead of the pontiff's visit (AFP)

Pope Francis used the word "genocide" on Sunday to describe the 1915 mass murder of Armenians in a move likely to severely strain diplomatic ties with Turkey.

"In the past century our human family has lived through three massive and unprecedented tragedies," he said during a solemn mass in Saint Peter's Basilica to mark the centenary of the Ottoman killings of Armenians.

"The first, which is widely considered 'the first genocide of the 20th century', struck your own Armenian people," he said, quoting a statement signed by Pope John Paul II and the Armenian patriarch in 2001.

Many historians describe the killings as the 20th century's first genocide, but Turkey hotly denies the accusation.

While Francis did not use his own words to describe the murders as genocide, John Paul II's use of the term provoked a sharp reaction from Turkey at the time, and citing the beloved former pope will again ruffle feathers.

- 'Immense and senseless slaughter' -

"We recall the centenary of that tragic event, that immense and senseless slaughter whose cruelty your forebears had to endure," Francis said.

He spoke of the duty to "honour their memory, for whenever memory fades, it means that evil allows wounds to fester."

The 78-year-old head of the Roman Catholic Church had been under pressure to use the term genocide publicly to describe the slaughter, despite the risk of alienating an important ally in the fight against radical Islam.

Armenians say up to 1.5 million of their kin were killed between 1915 and 1917 as the Ottoman Empire was falling apart, and have long sought to win international recognition of the massacres as genocide.

But Turkey rejects the claims, arguing that 300,000 to 500,000 Armenians and as many Turks died in civil strife when Armenians rose up against their Ottoman rulers and sided with invading Russian troops.

Over 20 nations, including France and Russia, recognise the killings as genocide.

- 'Shedding of innocent blood' -

In 2014, Turkey's President Recep Tayyip Erdogan, then premier, offered condolences for the mass killings for the first time, but the country still blames unrest and famine for many of the deaths.

Francis said the other two genocides of the 20th century were "perpetrated by Nazism and Stalinism", before pointing to more recent mass killings in Cambodia, Rwanda, Burundi and Bosnia.

"It seems that humanity is incapable of putting a halt to the shedding of innocent blood," he said.

The pope pointed to Armenia's particular importance as "the first Christian nation", being the first country to adopt Christianity as its state religion in 301 AD.

Those murdered a century ago were mainly Christian and although the killings were not openly driven by religious motives, the pontiff drew comparisons with modern Christian refugees fleeing Islamic militants.

He referred once again to the modern day as "a time of war, a third world war which is being fought piecemeal", and evoked the "muffled and forgotten cry" of those "decapitated, crucified, burned alive, or forced to leave their homeland."

"Today too we are experiencing a sort of genocide created by general and collective indifference," he said.