Pope Francis has denounced the mass killing of Armenians a century ago by Ottoman forces as "genocide", risking strong reaction from Turkey as he met Armenian President Serzh Sargsyan.

"This tragedy, this genocide, has unfortunately marked the start of a sad series of great catastrophes of the last century," the pontiff said at the presidential palace in Yerevan on Friday.

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The remark is the second time the Pope has referred to the killings as genocide, following a similar statement in 2015 which angered Turkey.

Turkey reacted furiously last year when Francis, during a mass St Peter's basilica, said that the massacres were "widely considered the first genocide of the 20th century".

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Ankara withdrew its ambassador from the Vatican in protest and relations remain deep frozen at a time when the Catholic Church is preoccupied by the plight of Christians in the Middle East, an issue in which Turkey is a key player.

Ankara agrees that many Armenians died in ethnic fighting and the deportation process between 1915 and 1917 during World War I, putting its estimate at 300,000 casualties. Armenia says 1.5 million died in the process in what it calls a "genocide".

Turkey has been further riled by Germany's decision, approved by parliament earlier this month, to recognise the killings as genocide.

Francis is the second pope to visit Armenia since it became an independent state following the collapse of the Soviet Union. Pope John Paul II went there in 2001.

The trip will also be the latest in a series of visits Francis has made to countries on the periphery of Europe where Catholics form a small minority of the population, following earlier trips to Albania and Bosnia.

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Source: Agencies