Pope Francis used the word "genocide" in referring to the mass killing of Armenians by Ottoman Turks during World War I during a Sunday Mass in St. Peter's Basilica on April 12, to commemorate the centenary of the incident, upsetting Turkey.

Turkey recalled its ambassador to the Vatican after the Pope used the word in reference to the killing of 1.5 million Armenians 100 years ago, Ireland's RTE news reported.

Francis made the comments at a Mass in the Armenian Catholic rite at St. Peter's Basilica, attended by the Armenian president and church leaders.

He said that humanity had lived through "three massive and unprecedented tragedies" in the last century, the BBC reported.

"The first, which is widely considered 'he first genocide of the 20th Century', struck your own Armenian people," he said, the BBC said citing words used by a declaration by the then Pope John Paul II in 2001.

His reiterated a joint statement issued by St. John Paul II and the head of the Armenian Apostolic Church in 2001 prompting Turkey to recalled the Vatican envoy in Ankara for "consultations," a report by CNN said.

In his homily, Francis explained it was important to acknowledge the incident so that there could be an appropriate closure for Armenians, especially those who lost relatives in the killings.

"Concealing or denying evil is like allowing a wound to keep bleeding without bandaging it," he said at the Mass, which coincided with Divine Mercy Sunday.

The Pope said he took it upon himself to honor the memory of Armenians who perished in the killings in the same manner as Catholic and Orthodox Syrians, Assyrians, Chaldeans and Greeks who "were murdered" by Ottoman Turks, who ruled before the modern State of Turkey was founded.

As the Ottoman Empire began to crumble, Turkish authorities drove  Armenians from their homelands. Armenia says the Turks planned and carried out the systematic killings of Armenians.

Turkey consistently denies the charge, saying the dead were victims of civil strife that occurred during the war. It also insisted that the numbers have been grossly inflated.

In a tweet, Turkish Foreign Minister Mevlut Cavusoglu criticized the Pope's remark saying it was very inaccurate.            

"The Pope's statement, which is far from historical and legal truths, cannot be accepted," Cavusoglu wrote. "Religious offices are not venues for instigating hatred and revenge with baseless accusations."

More than 20 nations, including France and Russia, have recognized the killings as genocide.