In a move likely to strain the Vatican's diplomatic relations with Turkey, Pope Francis referred to a mass killing of Armenians in the early 20th century as “genocide,” during a religious service to mark the 100th anniversary of the deaths.

"In the past century our human family has lived through three massive and unprecedented tragedies," Francis said during mass in Saint Peter's Basilica on Sunday morning. "The first, which is widely considered 'the first genocide of the 20th century', struck your own Armenian people," he said, according to the Jerusalem Post.

Armenia and many historians say as many as 1.5 million people were systematically killed by Ottoman forces in 1915, according to the BBC. Turkey, however, denies that the deaths represent genocide, saying that the death toll had been inflated, and that many of those killed were victims of partisan fighting.

The incident continues to cast a pall over relations between the two countries.

So far, reaction from the Turkish government has been muted. Turkey's embassy to the Holy See canceled a planned news conference for Sunday, presumably after learning that the pope would utter the word "genocide" over its objections, the Associated Press reported.

The Pontiff has close ties to the Armenian community from his days in Argentina, and said that it was his duty to honor the memory of those who were "senselessly" murdered.

"Concealing or denying evil is like allowing a wound to keep bleeding without bandaging it," he added.

In the service, the Pope also referred to the Holocaust and Stalinism, and mass killings in countries including Cambodia, Rwanda, Burundi and Bosnia that took place during the last century.

Francis is not the first Pope to describe the killings as genocide. Pope John Paul John Paul II used the word in a joint statement signed with the Armenian patriarch in 2000.

The move, however, provoked outrage in Turkey, and during a trip to the country the following year the then-Pontiff used the term “great evil” to describe the killings instead, according to RTE News.