~Christians flee attacks in northeast Nigeria~ MAIDUGURI, Nigeria - Hundreds of Christians have begun to flee northern Nigeria after dozens were killed in a series of attacks by Islamist militants who issued an ultimatum to Christians to leave the mainly Muslim region or be killed, witnesses said Saturday. A Nigerian newspaper Tuesday published a warning from Boko Haram, a movement styled on the Taliban, that Christians had three days to get out of northern Nigeria. Since the expiry of that ultimatum, attacks in towns in four states in northeastern Nigeria have left at least 37 people dead and hundreds of Christians are fleeing to the south, according to residents and a Red Cross official. Gunmen armed with Kalashnikovs have targeted church congregations and a group of mourners in a church hall. Witnesses said some shops run by Christians from the Igbo ethnic group in towns hit by the violence, including Yola and Mubi, were closed Saturday and residents started to pack their belongings onto buses heading to southern regions. There are fears of reprisal attacks on Muslims. Christian groups have asked their followers to remain peaceful but they concede that there is a risk of further violence. "We are very worried by the persistent killings. We have asked youths to remain calm. We stand for a united Nigeria but there is a limit to human tolerance," a spokesman for the Christian Association of Nigeria said.