By Philip Pullella and Margarita Antidze

YEREVAN (Reuters) - Pope Francis departed from his prepared text on Friday to use the word "genocide" to describe the mass killings of Armenians in 1915, a description that infuriated Turkey when he said it a year ago.

In an address to Armenia's president and the diplomatic corps, Francis used the Armenian term 'Metz Yeghern' (the great evil), but then added to his prepared text "that genocide" to refer to what he also called "the first of the deplorable series of catastrophes of the past century".

There was no immediate reaction from Turkey, which last year recalled its ambassador to the Vatican after the pope used the 'genocide' term. The envoy was kept away for 10 months.

Turkey accepts that many Christian Armenians living in the Ottoman Empire were killed in clashes with Ottoman forces during World War One, but it contests the figures and denies that the killings were systematically orchestrated and constitute a genocide. It also says many Muslim Turks perished at that time.

"There is no reason not to use this word in this case," Vatican spokesman Father Federico Lombardi told reporters. "The reality is clear and we never denied what the reality is."

Lombardi said the pope felt is was important for people to "understand the lessons of the past" and that it often seemed that little was learned from them.

He noted that the pope has in the past lamented that the killing of Armenians in 1915 was followed by the Stalinist purges in the Soviet Union in the 1930s, the Nazi attempt to exterminate the Jews, and more recently, the mass killings in the Balkans and the Great Lakes region of Africa.

After Friday night's unscripted remarks, which delighted his Armenian hosts, there was great anticipation for what he would say on Saturday morning when he visits Tzitzernakaberd, the "Genocide Memorial and Museum" in Yerevan.

(Reporting By Philip Pullella; Editing by Gareth Jones and Dominic Evans)