Liturgy in the Armenian Catholic rite held in St. Peter's Basilica

Pope Francis has called the slaughter of up to 1.5 million Armenians the 'first genocide of the 20th century."

In his remarks, Francis said the Armenian slaughter was the first of three "massive and unprecedented" genocides last century that was followed by the Holocaust and Stalinism. He said other mass killings had followed, including in Cambodia, Rwanda, Burundi and Bosnia.

The pontiff was speaking Sunday at the beginning of a Mass in the Armenian Catholic rite in St. Peter's Basilica in the Vatican, attended by dignitaries including Armenian President Serzh Sargsyan, to mark 100 years since the killings.

Francis said 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 said, according to the AP.

Francis cited a 2001 declaration signed by St. John Paul II and Armenian church leader Karenkin II that described the deaths as genocide, the AP reported.

The pope also pronounced St. Gregory of Narek — a 10th-century Armenian mystic — a doctor of the church, a title which has been given to only 35 people whose writings have greatly served the universal church, the news agency said.