Okah was convicted of 13 counts related to acts of terrorism.

He was arrested in Johannesburg a day after two car bombs exploded during the 50th anniversary of Nigeria's independence.

He had denied the charges, even though his militant group, Mend, said it had carried out the attack.

Johannesburg High Court Judge Neels Claassen convicted Okah on charges ranging from conspiracy to commit terrorism to detonating explosives.

"I have come to the conclusion that the state proved beyond reasonable doubt the guilt of the accused," Judge Claassen said, AFP news agency reports.

Okah was arrested on gun-running charges in Angola in 2007 and then transferred to Nigeria but never convicted.

He was released after two years under an amnesty for oil militants and he returned to South Africa, where he had lived since 2003.

Mend says it is fighting so that more of Nigeria's massive oil wealth is used to benefit the Niger Delta area which produces the oil.

But criminal gangs have taken advantage of the region's instability to make money from ransoms paid by oil companies, and by stealing oil.

The violence subsided significantly after the government offered militants an amnesty in 2009.

At its peak, the instability in the Niger Delta cost Nigeria about $1bn (£630m) in loss revenue, Reuters news agency quotes the central bank as saying.

Nigeria is Africa's biggest oil producer, but most of its people live in poverty.