It truly was Metz Yeghern, the 'Great Evil', as it is known by Armenians. Pope Francis

Pope Francis on Sunday called the slaughter of up to 1.5 million Armenians the "first genocide of the 20th century," prompting Turkey to recall its ambassador to the Vatican. USA Today reports.

Turkey, which denies a genocide took place, swiftly challenged the pope's comments to mark 100 years since the start of the killings, during a Mass in St. Peter's Basilica attended by Armenian President Serzh Sargsyan,

The Foreign Ministry said the Turkish people would not recognize the pope's statement "which is controversial in every aspect, which is based on prejudice, which distorts history and reduces the pains suffered in Anatolia under the conditions of the First World War to members of just one religion."

Francis, in his message to the Armenian faithful, said, "In the past century our human family has lived through three massive and unprecedented tragedies,"

"The first, which is widely considered 'the first genocide of the 20th century,' struck your own Armenian people, the first Christian nation, as well as Catholic and Orthodox Syrians, Assyrians, Chaldeans and Greeks," he said, citing a September 2001 declaration signed by St. John Paul II and Armenian church leader Karenkin II that described the deaths as genocide.

The pontiff also referred to the Holocaust and Stalinism, and mass killings in countries including Cambodia, Rwanda, Burundi and Bosnia.

Armenians have long campaigned for recognition that the killings, which happened between 1915 and 1917 under the rule of the Ottoman Empire, constituted genocide.

Armenia — which formally marks the killings on April 24 — and a number of historians say up to 1.5 million people died.

The pope's words strained relations with Turkey, which argues that the number of deaths has been inflated, and the people who died were victims of civil war and unrest during the collapse of the Ottoman Empire, not genocide.

In a message to all Armenians, the pope said: "A century has passed since that horrific massacre which was a true martyrdom of your people, in which many innocent people died as confessors and martyrs for the name of Christ.

"Even today, there is not an Armenian family untouched by the loss of loved ones due to that tragedy: it truly was Metz Yeghern, the 'Great Evil', as it is known by Armenians."

The killings are recognized as genocide by a number of countries around the world, but Turkey's allies Italy and the United States have avoided using the contentious term. The United Nations defined genocide as acts intended to destroy a national, ethnic, racial or religious group, in whole or in part.

Turkey's embassy to the Vatican canceled a planned news conference for Sunday, presumably after learning that the pope would utter the word "genocide" over its objections. Instead, the Foreign Ministry in Ankara issued a terse statement conveying its "great disappointment and sadness." It said the pope's words signaled a loss in trust, contradicted the pope's message of peace and was discriminatory because Francis only mentioned the pain of Christians, not Muslims or other religious groups.

The pope on Sunday also pronounced St. Gregory of Narek — a 10th-century Armenian monk and mystic — a doctor of the church, a title which has been given to just 35 other people.