Turkey has recalled its envoy to the Vatican after Pope Francis described the mass killing of Armenians under Ottoman rule in WW1 as "genocide".
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, and the row has continued to sour relations between Armenia and Turkey.
Turkey reacted with anger to the comments made by the Pope at a service in Rome on Sunday. They came at a Mass in the Armenian Catholic rite at Peter's Basilica, attended by the Armenian president and church leaders.
The Pope 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 took to Twitter to vent his feelings:
The Pope's statement, which is out of touch with both historical facts and legal basis, is simply unacceptable. +++— Mevlüt Çavuşoğlu (@MevlutCavusoglu) April 12, 2015
"Religious offices are not places through which hatred and animosity are fuelled by unfounded allegations," Mr Cavusoglu added.
In 2014, for the first time, Turkish PM Recep Tayyip Erdogan offered condolences to the grandchildren of all the Armenians who lost their lives.
But he also said that it was inadmissible for Armenia to turn the issue "into a matter of political conflict".
Armenia says up to 1.5 million people died in 1915-16 as the Ottoman empire split. Turkey has said the number of deaths was much smaller.
Most non-Turkish scholars of the events regard them as genocide. Among the other states which formally recognise them as genocide are Argentina, Belgium, Canada, France, Italy, Russia and Uruguay.
Turkey maintains that many of the dead were killed in clashes during World War I, and that ethnic Turks also suffered in the conflict.