Pope Francis has called the mass killing of Armenians under Ottoman rule in WW1 the 'first genocide of the 20th century."

The pontiff used the word “genocide” in a statement at the beginning of Sunday's Mass in the Armenian Catholic rite at Peter's Basilica.

The service was attended by Armenian President Serzh Sargsyan.

Pope Francis said on Sunday that humanity had lived through "three massive and unprecedented tragedies" in the last century.

"The first, which is widely considered 'the first genocide of the 20th Century', struck your own Armenian people," he said, as cited by the BBC News and the Associated Press.

Pope Francis used the wording of a 2001 declaration signed by St. John Paul II referring to the kiloings as the first genocide of the 20th century.

Armenia, which marks the start of the mass killings on April 24, says that up to 1.5 million Armenians were killed by Ottoman Turks around the time of World War I.

Turkey, however, has denied that the killings constituted genocide, arguing that the number of victims has been inflated, including victims of civil war and unrest during the collapse of the Ottoman Empire.