ABUJA (AFP) - Fear gripped Nigeria on Monday after a wave of Christmas bombings blamed on Islamists killed at least 40, including worshippers who were left begging for mercy and burning to death as they exited a church.

Hundreds of residents sought to flee the violence-torn city of Damaturu on Monday fearing further attacks and clashes between Islamists and police, while some 30 Christian shops were burnt in the nearby city of Potiskum late Sunday.

Nigeria has seen scores of attacks claimed by Islamist group Boko Haram, but some analysts said the bombings marked a dangerous escalation in a country divided between a mainly Muslim north and predominantly Christian south.

The government in Nigeria, Africa's most populous nation and its largest oil producer, blamed Islamist sect Boko Haram for three attacks on Sunday.

They included bomb explosions at two churches -- the deadliest as Christmas mass ended near the capital Abuja -- and a suicide attack in the northeast. A third church was targeted in the northeast on Christmas Eve, but no one was reported killed. Residents reported another explosion near a church in the northeastern city of Maiduguri late Sunday, but an army spokesman denied it.

The attack at St. Theresa Catholic Church in Madalla outside Abuja killed at least 35 and left a gruesome scene, with rescuers picking up body parts and putting them in plastic bags while emergency workers pleaded for ambulances. It struck as the service was ending and worshippers were filing out of the church. Some of the wounded, including one man whose entrails protruded from his body, ran toward a priest for final blessings. Some burnt in their cars as they sought to leave.

The attacks drew widespread condemnation, including from the Vatican, UN chief Ban Ki-moon, the United States and Britain. "I wish to express my solidarity with those who have been hit by this absurd act," Pope Benedict XVI told a crowd gathered for his Angelus prayer, adding that he was "deeply saddened" by the attacks.