(Updates with Cameroonian source, background)

By Estelle Shirbon

LAGOS, Nov 13 (Reuters) - Suspected Nigerian gunmen killed up to 21 Cameroonian soldiers in Bakassi, a border region that was handed back to Cameroon by Nigeria last year, Nigerian and Cameroonian sources said on Tuesday.

A senior Nigerian government source said no Nigerian armed forces were involved in the incident on Monday and Nigerian troops had also been attacked by gunmen in the same area within the last week.

"The Cameroonian gendarmes in Bakassi were attacked by an unknown armed group which killed 19, wounded six and made away with some arms and ammunitions," said the source, who did not wish to be named because the Nigerian Defence Headquarters were due to issue a statement later.

A Cameroonian military source in Yaounde, who asked not to be named, told Reuters the clash involved "a suspected militant group from Nigeria" and that 21 Cameroonian soldiers were reported killed in the fighting.

He added the attackers seized weapons and that eight Cameroonian soldiers were evacuated to Douala for treatment.

Local mayors in the towns of Kombo Itindi and Kombo Abedimo also reported recent fighting on the peninsula, but had no precise information about casualties.

The Nigerian government source said he had received reports of an attack on Nigerian naval troops in the coastal town of Ibeka in the same area within the last week. No lives were lost in that incident, he said.

Nigerian defence spokesman Solomon Giwa-Amu said the Cameroonian gendarmes could have been attacked by Nigerian criminals but details of the clash were unclear. He was certain no member of the Nigerian armed forces was involved.

"It's very unfortunate," he said.

"We are willing to cooperate with Cameroon to investigate this incident. All our resources are at their disposal to ensure that the criminals are brought to justice."

The disputed Bakassi peninsula, which includes offshore oil deposits, was a source of tension between the two countries for years until former Nigerian President Olusegun Obasanjo handed it over to Cameroon in 2006.

On the Nigerian side of the border, the oil-producing Niger Delta has been plagued by crime and militancy, with numerous gangs carrying out attacks on the oil industry, kidnappings and armed robberies.

Rebels demanding local control over oil revenues have blown up production facilities and kidnapped oil workers, while ransom seekers and extortion gangs have used the political struggle as cover to carry out crimes.

The Bakassi issue was separate from the Niger Delta crisis and since the handover of Bakassi, there has been no known case of an attack by Niger Delta criminals on Cameroonian troops. (Additional reporting by Tom Ashby in Lagos and Tansa Musa in Yaounde; Editing by Giles Elgood)

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