VATICAN CITY, July 7 - Pope Francis holds his first meeting with victims of sexual abuse by priests on Monday, an encounter that some say should have happened long ago, and victims from his native Argentina say they are pained over their exclusion. Six victims, two each from Ireland, Britain and Germany, will attend the pope's private morning Mass in his Vatican residence and then meet with him afterwards, according to people who organised the meeting. Francis has said he would show zero tolerance for anyone in the Catholic Church who abused children, including bishops, and compared sexual abuse of children by priests to a "Satanic Mass". But he has also come under fire from victims groups for saying in an interview this year that the Roman Catholic Church has done more than any other organisation to root out paedophiles in its ranks. Why the pope waited nearly 16 months since his election in March 2013 to meet with sexual abuse victims is not clear, particularly as his predecessor, former Pope Benedict, met several times with them during his trips outside Italy. "I think its very important that the pope meet with victims," said Anne Doyle of Bishops Accountability, a U.S.-based documentation centre on abuse in the Catholic Church. "We know that this pope is capable of compassion and his refusal to meet with sexual abuse victims so far has been inconsistent with the mercy he has shown with so many marginalised. This is something that he had to rectify," she said. Victims groups have said the pope had a spotty record of dealing with abuse cases in Argentina when he was archbishop of Buenos Aires, and victims from that country sent him a letter asking him why they were not invited.Pain of exclusion

This fact pains us," four victims of sexual abuse by priests said in a letter sent to the pope and made available to Reuters. "You must know the things that happen here and why the victims have been fighting for so many years, as well as the new cases that are surfacing," said the letter, signed by four victims. Doyle, of Bishops Accountability, said the pope should quickly follow up with "several core actions" to show that the meeting is not merely ceremonial. "He definitely must explicitly tell his bishops that all Church officials must report crimes and suspected crimes to civil authorities," pointing that in a number of developing countries it is up to the victim to report sexual crimes. The sexual abuse scandal has haunted the Catholic Church for over two decades but became a major issue in the United States about 10 years ago. Since then it has also disgraced local churches in Ireland, Germany, Belgium, the Netherlands and other countries and badly tarnished the Church's image. The Vatican says 3,420 credible accusations of sexual abuse by priests had been referred to the Vatican in the past 10 years and 824 clerics defrocked. The Church in the United States has paid $2.5 billion in compensation to victims. The commission advising the pope on the sexual abuse crisis, which includes Boston's Cardinal Sean O'Malley, met on Sunday and is expected to announce on Monday that it will expand it ranks to include more members from the developing world.