(CNN) -- Pope Francis risked Turkish anger on Sunday by using the word "genocide" to refer to the mass killings of Armenians a century ago under the Ottoman Empire. "In the past century, our human family has lived through three massive and unprecedented tragedies," the Pope said at a Mass at St. Peter's Basilica to commemorate the 100th anniversary of the Armenian massacres. "The first, which is widely considered 'the first genocide of the twentieth century,' struck your own Armenian people," he said, referencing a 2001 declaration by Pope John Paul II and the head of the Armenian church. His use of the term genocide -- even though he was quoting from the declaration -- upset Turkey. Turkey responded by summoning the Vatican ambassador for a meeting at the Foreign Ministry, Turkish state broadcaster TRT reported. In a tweet Sunday on his official account, Turkey's Foreign Minister Mevlut Cavusoglu called the Pope's use of the word "unacceptable" and "out of touch with both historical facts and legal basis." "Religious offices are not places through which hatred and animosity are fueled by unfounded allegations," the tweet reads. More than a million massacred Armenian groups and many scholars say that Turks planned and carried out genocide, starting in 1915, when more than a million ethnic Armenians were massacred in the final years of the Ottoman Empire. Turkey officially denies that a genocide took place, saying hundreds of thousands of Armenian Christians and Turkish Muslims died in intercommunal violence around the bloody battlefields of World War I. The Armenian government and influential Armenian diaspora groups have urged countries around the world to formally label the 1915 events as genocide. Turkey has responded with pressure of its own against such moves. Pope Francis said Sunday that "Catholic and Orthodox Syrians, Assyrians, Chaldeans and Greeks" were also killed in the bloodshed a century ago. He said Nazism and Stalinism were responsible for the other two "massive and unprecedented tragedies" of the past century.

CNN's Gul Tuysuz in Turkey and Karen Smith in Atlanta contributed to this report.