The two Ukrainians and two Norwegians released on Tuesday had been kidnapped last week.

Hafiz Ringim, the Bayelsa state police commissioner, said that they were handed over to authorities in the southern oil centre of Port Harcourt.

Five others were freed on Monday.

Five people - two Britons, two Germans and one American - are still missing.

Of the 14 expatriate oil-industry workers seized in the past ten days across the Niger Delta, nine have been freed unharmed.

'Hunt them down'

Obasanjo has called those behind the recent series of kidnappings "terrorists".

He said: "We are going to be firm and say no to violence and hostage-taking. Wherever we find hostage-takers now, we will hunt them down. We will not accept this any longer."

Hostage-taking and violence against the oil industry is common in the Niger Delta region, where much of the country's crude is pumped.

Despite the vast oil revenues in Africa's largest pumper of crude, most of the region's people remain impoverished.

Hostages are often used as bargaining chips by local communities to force payments or development schemes from Nigeria's federal government or the oil companies operating in the region.