President Goodluck Jonathan's face security woes at a time when his forces are stretched [EPA]

At least 50 people are reported to have been killed during clashes between rival ethnic groups in eastern Nigeria's Ebonyi state, the state government spokesman said.

The clashes erupted on Saturday from a long running rivalry between the Ezza and Ezilo people of Ishielu district that periodically flares up. Police said mobile units had been sent to the state to quell the violence.

"Up to 60 people died in the violence. It is difficult to give the exact figure because when we visited Ezilo community, which was the scene of the incident today ... villagers were still bringing out corpses.Fifty corpses were shown to us," Ebonyi state spokesman Onyekachi Eni told Reuters by telephone.

State Commissioner of Police Adeola Adeniji said the divisional crime officer for the area was killed in the violence.

Chioma Oke, an Ezilo who survived the clashes, said they started around 5 am.

"We heard sporadic gunshots and we began to run. They burned our houses. They said they were retaliating for an attack on them last year," she said.

There was no suggestion it had anything to do with wider security problems in the country stemming from a violent Islamist insurgency that set off a wave of deadly bombs on Christmas Day.

But they are likely to add to President Goodluck Jonathan's mounting security woes at a time when his forces are stretched.

Violent disputes over land are common in Africa's most populous country because the majority of its 160 million people are subsistence farmers living in rural areas with few means of arbitrating disputes.

Hundreds of people are killed every year in such clashes, many going unreported because of a lack of reliable information about them.