ABUJA (Reuters) - Gunmen attacked a vessel supporting a Transocean oil drilling rig in Nigeria's Niger Delta, wounding six people and seizing weapons and equipment, security sources said on Friday.

Three Nigerian workers were earlier reported to have been kidnapped in the attack, but it was later discovered that they had hidden under the boat's engine and reappeared unharmed.

"A security vessel, Mike One, supporting Trident 8, a Transocean rig drilling for Conoil in Brass, was attacked by armed men yesterday," said a security expert working for a major Western oil company operating in the Niger Delta.

Guy Cantwell, a spokesman for Transocean in Houston, said no hostages were taken but declined to give more details.

The six wounded were airlifted out and the Trident 8 rig has been shut down, the security sources said. No oil flow was affected.

Conoil is a Nigerian company.

Brass is in Bayelsa, one of three core oil-producing states in the Niger Delta, where kidnappings for ransom, attacks on oil facilities and armed robberies are common.

Nigeria is the world's eighth-biggest oil exporter but output has been reduced by 500,000 barrels per day for over a year after a wave of militant raids on Royal Dutch Shell oilfields in February 2006.

The sources said the vessel attacked on Thursday night was a 10-minute boat ride away from Don Walker, a rig operated by Shell, and staff there had requested security reinforcements from the nearest naval base.

Nigeria is due to hold elections for a new president and National Assembly on Saturday and there have been violent protests over widespread vote-rigging during last week's state polls.

The sources said the attack on the oil vessel appeared to be criminal rather than politically motivated.

"They robbed everything they could find on the vessel. It looks like a case of criminality," one of the sources said.