The incident was the first major kidnapping in southern Nigeria for months, following a lull in the wake of a government amnesty which saw thousands of rebels lay down their arms.

No group has as yet claimed responsibility for the abduction of the four men.

Ceasefire review

Armed groups claiming to seek a fairer share of oil revenue for local communities have since 2006 staged attacks on oil installations in the Niger Delta, playing havoc with crude output and international oil prices.

The Movement for the Emancipation of the Niger Delta (Mend), the main rebel group in the restive region, declared an open-ended ceasefire in October to give dialogue with authorities a chance.

But at the weekend, Mend said it was "reviewing its indefinite ceasefire announced on Sunday, October 9, 2009 and will announce its position on or before January 30, 2010".

The absence of Umaru Yar'Adua, the Nigerian president, for nearly two months for medical reasons has not helped efforts to end the violence in the Niger Delta with fighters reportedly unhappy at the slow progress of the process to re-train them and integrate them back into their communities.

Hundreds of foreign and local oil workers have been kidnapped in the Niger Delta since 2006. Many have been released unharmed, others after ransom payments.

Last year Shell said 152 of its workers, including 19 contractors, were kidnapped between 2006 and 2008.