Thousands of Christians are killed every year because of their faith and the persecution is becoming more widespread, Catholic observers said as Christmas celebrations are increasingly marred by attacks in at-risk countries.Pope Francis addressed the killings in a recent interview saying there was "an ecumenism of blood", meaning that Christians of all denominations including Catholic, Protestant and Orthodox are targeted."The narrative about Christianity in the Western mind is that a Christian is rich, powerful and has massive political influence," said John Allen, a Vatican expert and author of the new book "The Global War on Christians"."It doesn't reflect the reality. They are impoverished, linguistic and cultural minorities," he said.Estimates on Christian deaths are contested and vary widely.Out of around 2.3 billion Christians in the world, between 9,000 and 100,000 people are said to be killed because of their religion every year.In an editorial for Italy's top-selling Corriere della Sera daily on Monday, Andrea Riccardi, founder of the Sant'Egidio Catholic charity in Rome, criticised the "silence at Christmas over the persecutions".The US evangelical group Open Doors said the worst country for anti-Christian violence is North Korea, followed by Saudi Arabia, Afghanistan, Iraq, Somalia, Maldives, Mali, Iran, Yemen and Eritrea.The situation was said to be worsening in Egypt, Ethiopia and Syria as well.Open Doors has said that 100 million Christians are being persecuted -- a figure that has been widely criticised for including entire communities and not individual Christians at risk for their faith.Some observers say the reasons for anti-Christian violence are much more varied than simply a question of faith and are often linked to a variety of local issues.An upsurge in attacks in Central African Republic or South Sudan, for instance, is being seen as linked more to ethnic discord and battles for control of resources.The issue is a highly sensitive one, as shown by the criticism of Allen's talk of "war on Christians" by US professor Andrew Chestnut, an expert on Pentecostalism."Allen has not only done a disservice to Christians around the globe suffering real repression and persecution, but has dangerously fanned the flames of religious conflict," he wrote in the Huffington Post.Chestnut said that Allen had written about "a fictitious war that exists only on the pages and in the questionable data of certain Christian organisations".Experts have pointed to three main types of anti-Christian violence -- the main one being religious radicalism and particularly rising levels of Islamic as well as Buddhist and Hindu extremism.Allen said there were also cases of Christians being attacked by other Christians, like US evangelicals singled out by traditionalist Catholics in Mexico.The second threat is in states where governments see Christians as a possible subversive danger -- like in Eritrea or North Korea, where tens of thousands of Christians have been imprisoned, Allen said.There is also a persecution linked to businesses, mafia groups, paramilitaries or guerrillas that see Christian social teachings as a threat to their interests.