VATICAN CITY - Pope Francis will mark the 100th anniversary of the mass killings of Armenians with a special ceremony on Sunday, with all eyes on whether he will use the word "genocide".
The 78-year old is walking a diplomatic tightrope, pressured to use the term publicly to describe the Ottoman Turk murders, but wary of alienating a potentially key ally in the fight against radical Islam.
While many historians describe the cull as the 20th century's first genocide, the accusation is hotly denied by Turkey.
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.
Francis and Armenian patriarch Nerses Bedros XIX Tarmouni will celebrate a mass in Saint Peter's Basilica, which will include elements of the Armenian Catholic rite and be attended by the country's president Serzh Sargsyan.
The Vatican is holding the mass in time for those in attendance to return home for the official April 24 commemoration.
Using the word would not be a papal first: John Paul II used it in a joint statement signed with the Armenian patriarch in 2000 which said "the Armenian genocide, which began the century, was a prologue to horrors that would follow".
But it would be the first time the killings have been described as such during a mass in Saint Peter's Basilica.
'Annihilation of their brothers'
Before becoming pope, Jorge Bergoglio used the word several times in events marking the mass murders, calling on Turkey to recognise the killings as such, according to religious news agency I.Media.
As pope, Francis is said to only have used it in at one private audience in 2013 - but even that sparked an outraged reaction from Turkey.
During a meeting with a visiting Armenian delegation this week the pontiff deplored those "who were capable of systematically planning the annihilation of their brothers" - but stopped short of using the word genocide.
He called for "concrete gestures of peace and reconciliation between two nations that are still unable to come to a reasonable consensus on this sad event," saying both sides should be driven by the "love of truth and justice".
In 2014, Turkish 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.
Over 20 nations, including Italy, France and Russia, recognise the killings as genocide.
From the Greek word "genos", for race or tribe, and the suffix "cide" from the Latin for "to kill"; genocide is defined by the UN as an "act committed with intent to destroy in whole or in part, a national, ethnic, racial or religious group."
Religious observers say Francis, who stressed the importance of remembering "the martyrdom and persecution" of the Armenians, may make parallels in his homily to the rise in the persecution of Christians around the world.
Those murdered a century ago were mainly Christian and although the killings were not driven by religious motives, the pontiff has already drawn comparisons with modern Christians refugees fleeing Islamic militants.