A Nigerian security officer was killed in the incident, which occurred early on Tuesday as the expatriates travelled to work. No group has claimed responsibility.

"The abduction took place early this morning and four expatriates were taken, but we don't have any more details now," said Ibim Semenitari, spokeswoman for the Rivers State government.

Kidnappings for ransom are common in the oil-producing Niger Delta with hundreds of incidents reported each year. Most are released unharmed after a couple of days.

A security source, who did not want to be named, said the expatriates were contractors with Nigeria's National Engineering and Technical Company (NETCO) and working at the Afam power plant.

Company officials could not immediately be reached for comment.

Residents in Port Harcourt say violent crimes have become more common recently and blame it on former militants, who left the creeks of the Niger Delta as part of an amnesty offer that ended last October.

The amnesty offer is the most serious attempt yet to end years of unrest in the Niger Delta, where armed gangs have blown up pipelines and kidnapped foreign oil workers to push what they say are demands for a fairer share of the natural wealth.

But there have been delays to the promised monthly stipends and retraining programmes for those who agreed to disarm.

Tensions have heightened further due to the prolonged absence of President Umaru Yar'Adua which has stalled the post-amnesty programmes.

Yar'Adua has been absent from Nigeria for more than seven weeks receiving treatment for a heart condition in Saudi Arabia.