Two hostages seized by gunmen in separate incidents in Nigeria's Niger Delta this week were freed on Thursday, but security sources said another expatriate was kidnapped in the main oil city.

The Niger Delta, home to Africa's biggest oil and gas industry, has long been plagued by kidnappings but the number of attacks on vessels this year and abductions in areas previously considered relatively safe has alarmed the security forces.

A Romanian oil worker taken from an oil tanker off the coast of the Niger Delta on Wednesday was released early on Thursday, according to Lieutenant Colonel Sagir Musa, spokesman for the military task force in the southern state of Rivers.

His kidnappers used dynamite to attack the MT Meredith tanker as it travelled from the main commercial city Lagos to the oil hub of Port Harcourt laden with 4,000 tonnes of diesel.

It was the eighth such attack on shipping in waters off the Niger Delta this year.

Private security contractors working in the industry said they could not confirm the release of the Romanian.

In neighbouring Akwa Ibom state, the wife of an expatriate employee of U.S. energy giant ExxonMobil <XOM.N>, who was abducted by ransom-seekers on Wednesday, was freed by a security task force in the region, the authorities said.

"The victim was freed in that operation, which lasted a few hours following resistance by the deadly gang," Akwa Ibom state deputy governor Patrick Ekpotu said in statement.

Akwa Ibom had until recently largely escaped unrest plaguing the main Niger Delta states of Delta, Bayelsa and Rivers.

Ekpotu did not disclose the nationality of the woman, who was kidnapped in the town of Eket where ExxonMobil has offices, before being taken to a remote village.

EXPAT TAKEN FROM BUS

More than 200 foreigners have been kidnapped in the Niger Delta since the region's main militant group, the Movement for the Emancipation of the Niger Delta (MEND), began a campaign of sabotage against the oil industry just over three years ago.

Many have been taken by copycat criminal gangs and released unharmed after the payment of a ransom.

Security sources cited an eye witness saying armed men abducted an expatriate from a bus he was travelling in with colleagues in central Port Harcourt, the main oil city in the Niger Delta, on Thursday.

The sources said the military were in pursuit of the kidnappers but gave no further details.

Insecurity in the region has cut Nigeria's oil output by around a fifth and has led foreign companies in sectors from construction to telecoms, as well as the oil industry, to reconsider investments and reduce expatriate staffing.

MEND, which has been holding two British hostages for more than four months, warned almost two weeks ago that the kidnapping of "high-value oil workers" from Western Europe and North America would continue to be part of its strategy in 2009.

It said shortly after the attack on the MT Meredith that its affiliates were responsible but said it would ensure the Romanian hostage was released unharmed as soon as possible.