VATICAN CITY (AFP) - Pope Francis used the word "genocide" on Sunday to describe the 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 Turk killings of Armenians.

"The first, which is widely considered 'the first genocide of the 20th century', struck your own Armenian people," he said, citing a statement signed by John Paul II and the Armenian patriarch in 2000.

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

While many historians describe the cull as the 20th century's first genocide, Turkey hotly denies the accusation.

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 said the other two genocides of the 20th century were "perpetrated by Nazism and Stalinism."

"And more recently there have been other mass killings, like those in Cambodia, Rwanda, Burundi and Bosnia. It seems that humanity is incapable of putting a halt to the shedding of innocent blood," he added.