YENAGOA, Nigeria (Reuters) - Gunmen opened fire on a group of former militants in the oil producing Niger Delta late on Saturday, leading to a shootout that left eight people dead, a security official said.

None could confirm who carried out the attack. The men targeted were loyal to one of several Nigerian Delta militant leaders who accepted a government amnesty in 2009 in exchange for money, ending attacks that at one time shut down nearly half of production in Africa's biggest oil and gas industry.

Those killed included the younger brother of General Reuben Wilson, the leader of one former militant group, Wilson told Reuters at his house on Sunday where weeping mourners were gathered. He denied that his group had returned fired.

Tensions are growing between militant leaders enjoying the lucrative state handouts or government contracts and their foot soldiers who have been paid much less or received nothing, tensions that now threaten the Delta's fragile peace.

The shootout in the town of Lobia, which sits on the creeks and swamps of the Delta's Bayelsa state, was close to where an attack on a boat by disgruntled ex-militants killed 12 policemen in April.

"There was an armed collision between two groups at Lobia, which led to the death of an unspecified number of persons. The bodies are still in the river," said a spokesman for security forces in the Niger Delta, Lieutenant Colonel Onyeama Nwachukwu.

A senior security source, who also said the attack had led to a shootout, put the death toll at eight.

But Wilson denied that the group targeted by the unidentified assailants was armed.

"Unknown gunmen shot at them and killed them all and they were not armed, as claimed in some quarters," he said.

The motive was unclear. The previous attack in April was carried out by followers of Kile Selky Torughedie, who claimed Torughedie had embezzled amnesty money they were owed.

Any resurgence of Delta violence would be a blow to President Goodluck Jonathan, who helped negotiate the amnesty and who is from the Ijaw, the same ethnic group as most of the armed groups in the Delta. His forces are stretched by an Islamist insurgency in the north.

It would also be a problem for multinational energy companies such as leading operator Royal Dutch Shell that are already contending with industrial scale oil theft by armed gangs.

Militants claiming to be from the Movement for the Emancipation of the Niger Delta (MEND) claimed April's attack on a boat carrying police. Analysts doubted the claim as the group's commanders are mostly under amnesty and its leader, Henry Okah, is in jail.

(Reporting by Tife Owolabi; Writing by Tim Cocks; Editing by Tom Pfeiffer)