Nigerian authorities are negotiating with kidnappers for the release of seven foreign oil workers taken hostage in the Warri region on Thursday.
It is reported that at least one of the captives has already been freed.
The seven, who include two Colombians, a Briton, an Australian and a Russian, were testing an evacuation boat when they were captured.
Their captors, ethnic Ijaw youths, are demanding tens of thousands of dollars in ransom.
The kidnap is thought to have occurred as the men were testing the boat on the lake behind the plant.
Australia's junior foreign minister, Chris Gallus, said: I believe [they] were on a boat and there were shots, and the people who shot at them then asked them to come ashore and took them hostage."
The kidnappers initially demanded $330,000, the Nigerian Navy said, but subsequently reduced the demand to $33,000.
A BBC correspondent in Lagos, Anna Borzello, says kidnappings in the area are common and are usually carried out by militants who believe they do not benefit from the country's oil wealth.
Last week, 18 Nigerian oil workers were kidnapped by heavily armed militants but were subsequently released.
The British hostage, who has not yet been named, works for an Edinburgh-based pipeline coating company, Bredero Shaw.
The firm said one hostage had been released, and that the remaining hostages were being treated well and were unharmed.
Company spokesman Ted Scheffey said: "Communications are currently under way between the captors and the Nigerian officials to secure the release of the others."
He said he was confident an agreement would be reached.
"If you look at the past history in the area, typically they are resolved in a positive fashion," he added.
The UK Foreign Office told BBC News Online that consular staff were liaising with the oil company and local Nigerian officials.
"We can confirm a British national was among those kidnapped on 27 November, the next of kin are aware and are in regular contact with the employers," a spokesman said.
He could not confirm whether any of the hostages had been released.