About 90,000 people have been displaced in clashes between militant Islamists and the security forces in Nigeria's Damaturu city, an official has said.

The Boko Haram group and troops were involved in heavy gun battles in the north-eastern city last week.

The militants also planted bombs in Damaturu and outside a church near the capital, Abuja, on Christmas Day.

Meanwhile, an explosive device was thrown into an Arabic school in the south of country on Tuesday night.

At least seven children were wounded in the attack in Sapele town, police say.

The BBC Fidelis Mbah in the main city, Lagos, says it is not clear who was behind the explosion in Sapele, an oil-producing town in the Niger Delta.

On 10 December, a similar explosion happened at a mosque in the town, but it has raised fears about retaliatory attacks because of the Christmas Day violence, he says.

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I want to assure all Nigerians that there is no conflict between Muslims and Christians, between Islam and Christianity”

Muhammad Sa'ad Abubakar Sultan of Sokoto

It is highly unusual for explosions to happen in residential areas in the Delta, a region plagued by conflict between the government and militants demanding a greater share of the region's oil wealth.

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