Turkey has recalled its envoy to the Vatican after Pope Francis described the mass killing of Armenians under Ottoman rule in WW1 as "genocide".
Turkey has reacted with anger to the comment made by the Pope at a service in Rome earlier on Sunday.
Armenia and many historians say up to 1.5 million people were killed by Ottoman forces in 1915.
But Turkey has always disputed that figure and said the deaths were part of a civil conflict triggered by WW1.
The row has continued to sour relations between Armenia and Turkey.
The Pope made the comments at a Mass in the Armenian Catholic rite at Peter's Basilica, attended by the Armenian president and church leaders.
He said that humanity had lived through "three massive and unprecedented tragedies" in the last century.
"The first, which is widely considered 'the first genocide of the 20th Century', struck your own Armenian people," he said, in a form of words used by a declaration by Pope John Paul II in 2001.
Pope Francis also referred to the crimes "perpetrated by Nazism and Stalinism" and said other genocides had followed in Cambodia, Rwanda, Burundi and Bosnia.
He said it was his duty to honour the memories of those who were killed.
"Concealing or denying evil is like allowing a wound to keep bleeding without bandaging it," the Pope added.
Armenia's President Serzh Sargsyan welcomed his comments, saying they sent a powerful message to the international community.
But Turkey immediately summoned the Vatican's ambassador to Ankara for an explanation, and then later recalled its ambassador from Rome.
The foreign ministry said it felt "great disappointment and sadness" at the Pope's remarks, which it said would cause a "problem of trust" between them.
Turkey's Foreign Minister Mevlut Cavusoglu tweeted: "The Pope's statement, which is far from the legal and historical reality, cannot be accepted.
"Religious authorities are not the places to incite resentment and hatred with baseless allegations," he added.