By Estelle Shirbon ABUJA, Oct 4 (Reuters) - Militants freed around 25 kidnapped Nigerian oil workers on Wednesday but five abducted expatriates were still missing in another part of the Niger Delta after an unprecedented attack on a residential compound. ExxonMobil <XOM.N> and its contractors struggled to find out what had happened to the five foreigners, whose kidnapping on Tuesday brought new security worries to the Niger Delta. Suspected militants seized them after killing two Nigerian security guards and invading a residential compound for Exxon contractors in Eket, in Akwa Ibom state in the eastern delta. Security experts working for oil companies in the delta said it was the first time kidnappers had taken expatriates from within a residential compound. Such compounds are usually under tight security due to a history of abductions in the delta. The experts also said it was worrying this kind of trouble had spread to Akwa Ibom, which had been relatively quiet all year while militants staged a wave of attacks on oil facilities and kidnappings in three oil producing states further west. "We're quite concerned about it. The picture is not totally clear. We're trying to see how we can support the contractor companies in their efforts to find out what happened," said Paul Arinze, spokesman for the upstream division of Exxon. The Eket raid came two days after about 70 gunmen attacked a convoy of boats supplying oilfields operated by Royal Dutch Shell <RDSa.L> in neighbouring Rivers state. They killed at least three soldiers and abducted 25 Shell contractors. All the contractors had now been freed, a Shell company spokeswoman said on Wednesday, adding that no production was affected in the attack. SHOCK The incidents came as a shock after a relatively quiet September in the delta, which accounts for all crude output from the world's eighth-biggest exporter. A sixth of production capacity has been shut in since February following a wave of militant attacks that month, and in August 18 oil workers were kidnapped in eight separate incidents. All have been freed except for one Nigerian who was killed in a botched attempt to release him. Security sources said three Britons, one Indonesian and one Romanian were kidnapped at Eket. Earlier information suggesting two Malaysians were involved appeared to be wrong, they said. At least some of the men were thought to be employees of Bristow Goup Inc <BRS.N>, a helicopter services company. "We are aware that there has been an incident and Eket and we are investigating. We are taking it very seriously. It's possible that some of our staff may have been kidnapped but we can't confirm yet exactly what happened," said a Bristow official at the company's office in Britain. ExxonMobil has a residential compound of its own in Eket. The oil major, which exports about 800,000 barrels per day from Nigeria, operates the Qua Iboe Terminal, a major export facility, on the coast of Akwa Ibom. Supply disruptions from OPEC member Nigeria have contributed to several hikes in world oil prices this year. The latest incidents did not affect output as they did not occur at production facilities. Violence in the delta is rooted in poverty, corruption and lawlessness. Most inhabitants of the wetlands region almost the size of England have seen few benefits from five decades of oil extraction that has damaged their environment. Resentment towards the oil industry breeds militancy, but the struggle for control of a lucrative oil smuggling business and the lure of ransoms have also contributed to the violence.
