Nigeria’s authorities have ordered a 24-hour curfew in the central city of Jos after fighting resumed between gangs of Christian and Muslim youths.
Gunfire has been heard, while smoke is rising from burning buildings. At least 20 people are reported to have been killed since Sunday.
A doctor told the BBC that more bodies had arrived in the mortuary.
Extra troops have been deployed to the area, which has seen several bouts of deadly violence in recent years.
At least 200 people were killed in clashes between Muslims and Christians 2008, while some 1,000 died in 2001.
Houses, mosques and churches were set alight on Sunday.
At least 3,000 people have fled their homes, according to the Red Cross.
Residents say many buildings have been set on fire, especially in the northern parts of the city.
“As early as 4 am (0300 GMT), we started hearing gunshots and machine-gun fire and this has gone on for hours,” Dr Aboi Madaki, who works at the Jos University Teaching Hospital, told the Reuters news agency.
“I saw soldiers moving into town and I can see smoke coming from many places.”
Plateau State information commissioner Gregory Yenlong said a dusk-to-dawn curfew had now been extended.
“All residents are hereby directed to stay indoors as security agents work towards restoring peace,” he told the AFP news agency.
The Plateau State authorities have said that more than 60 arrests have been made but they have not said how many people have died since Sunday.
Plateau State spokesman Dan Manjang said it was not yet known what had sparked the unrest.
He told the BBC’s Network Africa programme there were reports that it may have started after a football match – although he said that would surprising.
Reuters news agency quoted residents as saying the violence started after an argument over the rebuilding of homes destroyed in the 2008 clashes.
Nigerian Red Cross official in Jos Awwal Madobi told the BBC that many families had fled the violence.
“Some are in the church, some in the mosque and the NDLEA (Nigeria Drug Law Enforcement Agency),” he said.
“It’s not that they are directly affected but they are scared and want to be somewhere secure for their safety.”
He said they needed blankets and food as they had fled empty-handed.
Jos is in Nigeria’s volatile Middle Belt – between the mainly Muslim north and the south where the majority is Christian or follows traditional religions.
Correspondents say such clashes in Nigeria are often blamed on sectarianism, however poverty and access to resources such as land often lies at the root of the violence.