Pope Francis has angered the Turkish government by describing the mass-murders of up to 1.5million Armenians in 1915 as 'the first . genocide of the 20th century'. The pontiff made the comments at a 100th anniversary Mass on . Sunday, prompting Turkey to summon the Holy See's ambassador in . Ankara in protest. Turkey told the Vatican ambassador it was 'deeply sorry and disappointed' in Pope Francis, adding that his comments had caused a 'problem of trust'. Scroll down for video . Turkey said Pope Francis has caused a 'problem of trust' after describing the mass-murders of up to 1.5million Armenians by Ottoman Turks in 1915 as 'the first genocide of the 20th century' While Turkey accepts that many  Armenians died in . clashes with Ottoman soldiers beginning in 1915, when Armenia . was part of the empire ruled from Istanbul, it denies that the victims reached the estimated 1.5million and that this amounted to genocide. Today was the first time a pope has publicly used  'genocide' to describe the massacre, although it is a term used by many European and South American governments. In 2001, Pope John Paul II and Armenian Apostolic Church . Supreme Patriarch Kerekin II called it 'the first genocide of . the 20th century' in a joint written statement. Francis, who has disregarded many aspects of protocol since . becoming pope two years ago, uttered the phrase during a private . meeting at the Vatican with an Armenian delegation in 2013, . prompting a strong protest from Ankara. As the archbishop of Buenos Aires before becoming the leader . of the world's 1.2 billion Catholics, Jorge Maria Bergoglio had . already publicly characterised the mass killings as genocide. Pope Francis listens as the Patriarch of the Armenian Catholic Church Nerses Bedros XIX Tarmouni speaks during a mass on the 100th anniversary of the Armenian mass killings in St. Peter's Basilica at the Vatican . Mass: Pope Francis greets the Head of Armenia's Orthodox Church Karekin II, during an Armenian-Rite Mass on the occasion of the commemoration of the 100th anniversary of the Armenian Genocide . In memoriam: Prelates take pictures as Pope Francis during the mass in the Vatican today . In November, the Argentine-born pontiff made an official . visit to Turkey as part of his efforts to solidify relations . with moderate Muslim states. This April marks the 100th anniversary of the massacre of an estimated 1.5 million Armenians by Ottoman Turks. The anniversary of the mass killings in World War One will be commemorated by Armenia on April 24. The killings in 1915 are regarded by many historians as the first genocide of the 20th century, and are said to have inspired Nazi leader Adolf Hitler. Armenians say up to 1.5 million of their kin were killed between 1915 and 1917 as the Ottoman Empire was falling apart and have long sought to win international recognition of the massacres as genocide. Turkey rejects the claims, arguing that 300,000 to 500,000 Armenians and as many Turks died in civil strife when Armenians rose up against their Ottoman rulers and sided with invading Russian troops. Several European countries recognize the massacres as such, though Italy and the United States, for example, have avoided using the term officially given the importance they place on Turkey as an ally. Last year, Turkish President Recep Tayyip Erdogan issued a message of condolences to descendants of Armenians killed and said Turkey was ready to confront the history of the killings. More recently, Erdogan has accused Armenians of not looking for the truth but seeking to score points against Turkey, saying numerous calls from Turkey for joint research to document precisely what happened had gone unanswered. At the start of the Armenian rite Mass in St. Peter's . Basilica, Pope Francis described the 'senseless slaughter' of . 100 years ago as 'the first genocide of the 20th century', which . was followed by 'Nazism and Stalinism'. 'It is necessary, and indeed a duty, to honour their memory, . for whenever memory fades, it means that evil allows wounds to . fester. Concealing or denying evil is like allowing a wound to . keep bleeding without bandaging it!' he said. Francis's comments were also published by Armenian President . Serzh Sargyan's office on Sunday. 'We are deeply grateful to His Holiness Pope Francis for the . idea of this unprecedented liturgy ... which symbolizes our . solidarity with the people of the Christian world,' Sargyan said . in a speech at a Vatican dinner on Saturday evening. After Francis's remarks on Sunday, Turkey swiftly summoned . the Vatican's ambassador in Ankara to protest and seek an . explanation. 'The pope's statement which is far from historic and legal truths is unacceptable. Religious positions are not places where unfounded claims are made and hatred is stirred,' Foreign Minister Mevlut Cavusoglu tweeted. The Foreign Ministry in Ankara later issued a 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. Francis also urged reconciliation between Turkey and . Armenia, and between Armenia and Azerbaijan over the disputed . Caucasus mountain region of Nagorno-Karabakh. The appeal came in . a letter handed out during a meeting after the Mass to Sargyan . and the three most important Armenian church patriarchs present.
Pope calls mass murder of Armenians 'first genocide of the 20th century'
The 1915 killings saw 1.5m Armenians slaughtered by Ottoman Turks .
Turkey said Pope Francis'  comments had caused a 'problem of trust'
Turkey denies killings were genocide, saying both sides suffered loss .