Turkey has criticised Pope Francis for using the word "genocide" to describe the mass killing of Armenians under Ottoman rule in World War 1.
Ankara immediately summoned the Vatican's envoy after the Pope made the comments at a service in Rome.
Turkey's Foreign Minister described it as "far from the historical reality".
Armenia and many historians say up to 1.5 million people were killed by Ottoman forces in 1915. Turkey has always disputed the number of dead.
The dispute has continued to sour relations between Armenia and Turkey.
'Bleeding wound'
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.
Turkey said it summoned the Vatican's ambassador to Ankara, Archbishop Antonino Lucibello, to seek an explanation over the comments.
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.
"The Pope's statement, which is far from the legal and historical reality, cannot be accepted," tweeted Turkey's Foreign Minister Mevlut Cavusoglu.
"Religious authorities are not the places to incite resentment and hatred with baseless allegations," he added.
Pope Francis, who visited Turkey last year, would have been perfectly conscious that he would offend the moderate Muslim country by his use of the word "genocide".
But the Pope's powerful phrase "concealing or denying evil is like allowing a wound to bleed without bandaging it" extended his condemnation to all other, more recent, mass killings.
It now remains to be seen how far his remarks will impact upon the Vatican's future relations with moderate Muslim states. It was a bold decision but totally coherent with Pope Francis' philosophy of open discussion about moral arguments.
Pope Francis' focus today on Armenia, the first country to adopt Christianity as its state religion, even before the conversion of the Roman Emperor Constantine, serves as yet another reminder of the Catholic Church's widely spread roots in Eastern Europe and the Middle East.
more
Turkey anger at Pope Francis Armenian 'genocide' claim - BBC News