(Adds staff evacuated from rig, paragraph 4)

By Austin Ekeinde

PORT HARCOURT, Nigeria, May 5 (Reuters) - Gunmen abducted a British oil worker from a U.S.-owned drilling rig off Nigeria's coast on Saturday, triggering a security alert at a nearby oil export terminal, authorities said.

The Briton was taken from the Trident 8 rig operated by U.S.-based Transocean <RIG.N> off the coast of the state of Bayelsa, in the latest in a string of abductions that have disrupted oil supplies from the world's eighth largest exporter.

The attack triggered a security lockdown at the nearby Brass crude oil export terminal, which exports 200,000 barrels per day, operated by Italian oil company Agip <ENI.MI>, security sources said.

"There has been one Transocean sub-contractor taken from the Trident 8 rig," said Transocean spokesman Guy Cantwell in Houston, adding that the remaining 23 staff were safe and had been taken off the rig.

Security sources at first reported two foreign workers abducted from the rig and that the Brass terminal was also attacked, but later said the second missing person had resurfaced after hiding in his cabin and that the Brass terminal had not been directly targeted.

Agip's parent company ENI <ENI.MI> said there was no confirmation of an attack on any of its plants in Nigeria.

The Trident 8 rig is exploring for oil for Nigerian company Conoil <NTOL.LG>. No oil flows were affected.

The latest abduction brings to 27 the number of foreign workers kidnapped in the world's eighth largest oil exporter in three attacks this week. Eight were released and 19 are still being held.

Militants fighting for more autonomy in the oil-producing Niger Delta have stepped up a campaign of attacks and kidnappings on Western oil facilities in Nigeria, but the line between militancy and crime is blurred and most abductions are motivated by groups seeking ransom.

Hostages are almost always treated well and released unharmed, although a few have been killed by Nigerian troops in clumsy rescue attempts.

Thousands of foreign oil workers have fled Nigeria since a string of militant attacks in Feb. 2006 that reduced output by 600,000 barrels per day, or one fifth of total capacity.

Another 65,000 barrels per day were shut off by the attacks earlier this week. (Additional reporting by Tom Ashby in Lagos)

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