Concern over four oil workers held hostage in Nigeria grew last night after their captors warned that one was gravely ill and that fresh attacks on oil installations were imminent.

The Movement for the Emancipation of the Niger Delta, a militant rebel group, seized the Royal Dutch Shell employees from a company vessel last week during a series of assaults on pipelines and flow stations which drove up oil prices.

The hostages, an American, a Briton, a Bulgarian and a Honduran, have been kept on the move in the Niger delta's maze of malaria-infested swamps and tidal creeks.

Patrick Landry, the American, suffers high blood pressure and could soon die, a kidnapper, who described himself as a commander, told Reuters by phone on Thursday. "One of them is sick, badly sick, and could give up tonight. If one of them dies, we kill them all."

The statement contradicted previous assurances that the captives would remain unharmed as long as the Nigerian army did not attempt a rescue. It also denied Nigerian government claims that negotiations were under way.

Mr Landry, permitted to make a telephone call to friends and colleagues, said: "We are in bad shape, we really are. Meet these people's demands. We are not military: we came here to work."

Nigel Watson-Clark, the British hostage, read out the militants' demands, which included compensation for pollution, the release of two ethnic Ijaw leaders and greater local control over the region's oil wealth.

"I have got to tell you we are under a lot of pressure here and things aren't too good," said Mr Watson-Clark. He urged Britain to lobby the Nigerian government for their release.

In an email statement yesterday, the rebels gave no update about the hostages but warned of widening their attacks against other oil companies, whom they accuse of collaborating with a corrupt government in siphoning off the region's wealth.

"Be assured that we will continue our attacks very shortly. We are more than capable of sustaining the conflict."

Since clashes flared last month dozens have been reported killed. Shell has withdrawn 500 workers and cut production by 210,000 barrels a day, about a tenth of national output. Nigeria is the world's eight-largest oil exporter.

In a joint statement the country's two main oil workers' unions threatened to withdraw their members if security was not restored.

To escape the delta's chronic instability, oil companies are moving operations offshore.