Pope Francis on Sunday called the 1915 killings of up to 1.5 million Armenians under the Ottoman Empire a "genocide," upsetting Turkey, which has long sought to downplay the massacre, CNN reported.

The leader of the world's more than 1 billion Catholics minced no words when he celebrated a Mass at St. Peter's Basilica to commemorate the 100th anniversary of the atrocity.

"In the past century, our human family has lived through three massive and unprecedented tragedies," the pontiff noted. "The first, which is widely considered 'the first genocide of the 20th century,' struck your own Armenian people," Francis added.

The Church leader was quoting a 2001 joint declaration by Pope John Paul II and the head of the Armenian church; nevertheless, his use of the term "genocide" was not well-received in Ankara, the news channel said.

Turkey, which denies a genocide took place and views those killed as victims of civil war and unrest, swiftly responded by recalling its ambassador to the Holy See and accusing Francis of spreading hatred and "unfounded claims," according to the Associated Press.

Turkish Foreign Minister Mevlut Cavusoglu strayed from diplomatic protocol when he took to Twitter to attack the pontiff, who as Vatican City monarch is also a head of state.

"The pope's statement, which is far from historic and legal truths, is unacceptable," Cavusoglu said, according to USA Today. "Religious positions are not places where unfounded claims are made and hatred is stirred," the foreign minister added.

Ankara also summoned the apostolic nuntius -- the Vatican equivalent of an ambassador -- for a meeting, CNN said based on reports by Turkish state broadcaster TRT.

Armenian Foreign Minister Edward Nalbandian, meanwhile, said he was grateful to Francis, who hails from Buenos Aires, a city with a large and influential Armenian immigrant population. Nalbandian chided Turkey for its isolated stance on the 1915 massacre.

"We are in a situation in which Turkey speaks a different language from the rest of the international community and it seems that it doesn't understand that it is speaking a different language," he told the Italian news agency Adnkronos, CNN added.