VATICAN CITY - Pope Francis described the massacre of as many as 1.5 million Armenians as "the first genocide of the 20th century" at a 100th anniversary Mass on Sunday, choosing words that could draw an angry reaction from Turkey.
Muslim Turkey accepts that many Christian Armenians died in clashes with Ottoman soldiers beginning in 1915, when Armenia was part of the empire ruled from Istanbul, but denies hundreds of thousands were killed and that this amounted to genocide.
It was the first time a pope has publicly pronounced the word "genocide" for the massacre, repeating a term used by some European and South American countries but avoided by the United States and some others to maintain good relations with an important ally.
Pope John Paul II and Armenian Apostolic Church Supreme Patriarch Kerekin II called it "the first genocide of the 20th century" in a joint written statement in 2001.
Francis, who has disregarded many aspects of protocol since becoming pope two years ago, uttered the phrase during a Vatican audience with an Armenian delegation in 2013, prompting a strong protest from Ankara.
At the start of the Armenian rite Mass in St. Peter's Basilica, the Argentine-born pontiff described the "senseless slaughter" of 100 years ago as "the first genocide of the 20th century", which was followed by "Nazism and Stalinism".
"It is necessary, and indeed a duty, to honor their memory, for whenever memory fades, it means that evil allows wounds to fester. Concealing or denying evil is like allowing a wound to keep bleeding without bandaging it!" he said.
Turkey has summoned the Vatican's ambassador in Ankara over the pope's comments, and the Turkish foreign ministry was expected to issue a statement on his use of the word "genocide" later on Sunday.
Francis's comments were published by Armenian President Serzh Sargyan's office on Sunday.
"We are deeply grateful to His Holiness Pope Francis for the idea of this unprecedented liturgy ... which symbolizes our solidarity with the people of the Christian world," Sargyan said in a speech at a Vatican dinner on Saturday evening.
The pope said genocide continues today against Christians "who, on account of their faith in Christ or their ethnic origin, are publicly and ruthlessly put to death - decapitated, crucified, burned alive - or forced to leave their homeland."
Islamic State insurgents have persecuted Shi'ite Muslims, Christians and others who do not share their ultra-radical brand of Sunni Islam as they carved a self-declared caliphate out of swathes of Syria and Iraq, which share borders with Turkey.
Francis also urged reconciliation between Turkey and Armenia, and between Armenia and Azerbaijan over the disputed Caucasus mountain region of Nagorno-Karabakh. The appeal came in a letter handed out during a meeting after the Mass to Sargyan and the three most important Armenian church patriarchs present.