The four hostages read out their captors' demands by telephone on Monday and warned against attempted rescue.
The kidnappers have staged a series of attacks on oil pipelines, platforms and workers over the past three weeks, denting supply from the world's eighth-largest exporter and driving up world prices.
Four soldiers and eight rebels were killed in a firefight on Sunday at Shell's Benisede platform, Brigadier-General Elias Zamani, who leads a military task force of thousands in the delta, said.
Two attacks last week hit Nigeria's oil output by 226,000 barrels per day, or 10%, but it has since partially recovered to leave a 5% shortfall.
The rebels have vowed to stop all oil exports from Nigeria and advised workers to leave the delta or die.
The rebels have been attacking
oil infrastructure and workers
The possibility of a major Shell staff withdrawal raises the prospect of further output cuts and will increase pressure on the government to crack down on the rebels.
One of the hostages, identifying himself as Nigel Watson-Clark from Britain, read out a list of five demands by the Movement for the Emancipation of the Niger Delta, with a 48-hour deadline.
The group demanded local control of the Niger Delta's oil wealth, payment of $1.5 billion by Shell to the Bayelsa state government to compensate for pollution, and the release of three men, including two ethnic Ijaw leaders, the British hostage said.
"If the Nigerian government does not meet these demands in 48 hours, whatever happens is in their own doing," he said, asking Britain to put pressure on Nigeria to negotiate.
Warning against intervention
The four hostages - a Bulgarian, an American, and a Honduran besides the Briton - said in what appeared to be prepared statements that they were being treated well, but that any attempted military intervention or rescue could cost them their lives.
A man who identified himself as Harry Ebanks from Honduras, said: "I'd like to contact my family and let them know that I am all right and everything with us is good.
"The only thing is the environment is not good with us because there is a lot of mosquitoes and it is dangerous for us."
"The Nigerian government should not make any military intervention. Okay? They should not make any attempt to rescue us as it has been made clear that it may result in the loss of our lives"
Patrick Landry, the American hostage, said: "No military intervention is a must. This climate in the conditions we're in is not conducive to us, especially as I am an older man and my health is not good."
Watson-Clark said: "The Nigerian government should not make any military intervention. Okay? They should not make any attempt to rescue us as it has been made clear that it may result in the loss of our lives."
Analysts say the violence is part of growing regional rivalry in Nigeria, Africa's most populous nation, ahead of presidential elections in 2007.
Violence against oil workers is frequent in the Niger Delta, which accounts for almost all of Nigeria's 2.4 million barrels per day output and where an estimated 20 million people live in poverty alongside a multibillion-dollar industry.
Much of the rhetoric of rebel Niger Delta groups is echoed by regional politicians, who have demanded a greater share of the oil wealth.