LAGOS (AFP) – The family of a notorious ex-militant in Nigeria has told police he was killed in an ambush that has sparked fears of violence ahead of upcoming presidential elections, authorities said Thursday.

"Chief Adiele George, a member of Soboma George's family in Buguma, told us that his brother was killed on Tuesday," Rivers State police spokeswoman Rita Abbey told AFP.

But she said neither the police nor the family had seen George's corpse. The family was told by George's followers that he was shot in the head and chest by his attackers in the oil hub of Port Harcourt.

Abbey said it was believed that either his followers or his assailants had taken away the body of George, who had signed up to a government amnesty for Niger Delta oil militants only months ago.

"We are still investigating the motive for the killing," she said, adding that no arrests had been made.

Abbey said police had deployed armoured vehicles to flashpoints in Port Harcourt.

"Our men are on high alert to prevent any reprisal attacks," she said.

George was accused of mobilising his followers to intimidate voters and help rig elections in 2007, and his suspected death has led to fears of fresh violence in Port Harcourt.

Presidential, legislative and state elections are expected in January.

George's suspected death has likewise sparked concerns over the amnesty, which has been credited with bringing relative peace to the oil-producing Niger Delta after years of attacks and kidnappings.

George, the leader of the banned Outlaws group which terrorised Port Harcourt before the recent government amnesty, was believed killed along with a woman as he made his way out of a football field Tuesday evening.

Another woman who was seriously wounded in the attack was being treated in hospital, police said.

George was among those who accepted the amnesty granted by late president Umaru Yar'Adua for Niger Delta "oil rebels" after surrendering their arms between August and October last year.

Violence in the region between 2006 to 2009 played havoc with Nigeria's oil production, which dropped from 2.6 million barrels a day to about one million at the peak of the unrest.

But the amnesty has resulted in relative peace in the region, and Nigeria, one of the world's largest oil exporters, currently produces around 2.1 million barrels daily.