* Religious attacks kill dozens of Christians
* Boko Haram claim some attacks, hundreds flee northeast
* Fear of reprisals, sectarian conflict
(Recasts, adds quotes, details, background)
By Ibrahim Mshelizza
MAIDUGURI, Nigeria, Jarn 7 (Reuters) - 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 on Saturday.
A Nigerian newspaper on 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 on 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 told Reuters.
President Goodluck Jonathan declared a state of emergency in the northeast and two other regions in Nigeria on Dec. 31, in a bid to contain a growing insurgency by Boko Haram, which says it wants to apply Islamic sharia law across the country.
Heavily armed troops and tanks have been patrolling parts of northeast Nigeria since Jonathan made the announcement but it is a vast, remote region that has proven difficult to secure.
WAVE OF ASSAULTS
Gunmen opened fire in a hall in Mubi in Adamawa state on Friday where a group of Christians had gathered to mourn the deaths of those killed in an attack the previous day. The death toll in those attacks has reached 21, the Red Cross said on Saturday.
"Unknown gunmen in Mubi attacked and killed 3 people on Thursday night and on Friday as people gathered to mourn the deaths, the gunmen believed to be the same attackers killed 18 people, totalling 21," said Red Cross spokesman Umar Mairiga.
Adamawa state is just south of Borno state, the homeland of Boko Haram, which has been behind almost daily attacks in recent months.
Local residents in the Adamawa state capital Yola said gunmen had fired on Christians leaving church on Friday, killing eight people. The police confirmed the incident but were not able to give further comment or a death toll.
A spokesman for Boko Haram told reporters by phone that the sect was behind many of the attacks, including a shooting at a church service in northeast Gombe on Thursday, which killed at least six people.
"The Gombe attack on the Deeper Life Church and the attack on Igbos in Mubi and that of Yola were all carried out by us," Abu Qaqa said by telephone to reporters.
Elsewhere, a Christian couple were shot dead on Friday in the Mairi ward of Maiduguri, the capital of Borno state and the nucleus of Boko Haram's violence since an uprising in 2009.
"A Christian husband and wife have been killed in the night (Friday) in Maiduguri," said Colonel Victor Ebhemele, operations officer in the Borno joint task force.
In Yobe state, which sits on borders with Borno state and neighbouring Niger, police said it killed some members of Boko Haram in a gun battle on Friday night.
The Red Cross official said members of the Igbo ethnic group, who are usually Christian and a minority in the mainly Muslim north were fleeing the northeast. Most of the people killed in Mubi were Igbo, local residents said.
Boko Haram, whose name means "Western education is forbidden", claimed responsibility for a series of bomb attacks across Nigeria on Christmas Day, including one at a church near the capital Abuja that killed at least 37 people and wounded 57.
Nigeria's population of around 160 million is split roughly equally between Christians and Muslims. Most Christians live in the south and most Muslims in the north, but many communities are mixed, and the majority live side by side in peace.
The persistent violence adds to growing problems for Jonathan, who has been criticised for not getting a grip on Boko Haram's insurgency. Nationwide strikes are planned on Monday against the government's decision to end fuel subsidies from Jan. 1, which caused the pump price to double. (Reporting by Ibrahim Mshelizza and Joe Brock; Writing by Joe Brock; Editing by Rosalind Russell)