A spokesman from Bayelsa state in southern Nigeria, which is leading government contacts with the kidnappers, earlier said negotiators had reached a point where the foreigners - an American, a Briton, a Bulgarian and a Honduran - could be released at any time.
But an email believed to be from the militants said: "I promised you the hostages were going nowhere in spite of the rumours and repeat that to you."
A series of attacks on oil facilities over the past two weeks has killed at least 22 members of the security forces and three Nigerian oil workers, and shut down more than eight percent of the country's oil production.
The crisis in Africa’s biggest oil exporter has also helped push world oil prices up.
And in a sign that authorities have been taken aback by the ferocity of the latest round of violence, the Nigerian police announced that officers would be sent for training in "guerrilla warfare".
News agency AFP said an email believed from the kidnappers dismissed talk of an imminent release and insisted that the hostages would not be freed until their demands are met.
"Those guys are not going anywhere. The Nigerian government still fails to understand that this is different," said a statement from an e-mail account that has previously been used by the kidnap gang.
"They will not be released for any reason other than that specified in all our statements," the message warned.
"We are not discussing with anyone for a while. This is in reaction to the Nigerian government's non-appreciation of the situation at hand," the message said.
Photograph released
Meanwhile, a photograph released by Nigerian security sources showed British security expert Nigel Watson-Clark, Honduran engineer Harry Ebanks, US boat skipper Patrick Landry and their Bulgarian colleague Milko Nichev.
The four were kidnapped on January 11 by heavily armed ethnic Ijaw militants who stormed their supply vessel, the Liberty Service, as it worked off the delta coast in the EA offshore oil field for energy giant Shell.
The photograph, thought to be several days old, showed the men sitting on plastic chairs in a grove of oil palms guarded by three Nigerian guerrilla fighters, one with an assault rifle. The captives appeared to be uninjured.
The gang has demanded US$1.5 billion from Shell to compensate Ijaw communities polluted by the oil industry and the release of two local ethnic leaders being held by Nigerian authorities.
Various e-mailed statements identify the hostage-takers as hard-line elements of the 14-million-strong Ijaw ethnic group seeking to seize control of the oil resources on their land in the delta region.
Since the kidnapping, armed gangs have blown up a major oil pipeline and attacked two oil facilities, killing a total of 22 police and soldiers and three Nigerian oil workers. It is not clear if the attacks are linked.
Shell has cut production by 221,000 barrels since the start of the crisis and has warned that tankers arriving to pick up crude at its Forcados export terminal might have to wait for up to two weeks to load.
Oil prices on Friday surged past US$67 for the second time since the start of the crisis, after the militants threatened more attacks on foreign workers.
Nigeria is Africa's biggest oil exporter, producing 2.6 million barrels per day and accounting for 10 percent of the United States' oil imports.