A man identifying himself as US hostage Cody Oswald told news agency AFP by telephone that he had not seen his two colleagues for two days, after his captors separated the hostages.
In the same call, a rebel commander warned that the hostages would not be freed until the Nigerian federal government had pulled all of its troops out of the Niger Delta and released three prominent ethnic Ijaw leaders.
The rebels are seeking autonomy for their oil-rich region.
The caller, who identified himself as a commander in the Movement for the Emancipation of the Niger Delta (MEND), also demanded that the oil giant Shell pay US$1.5 billion in damages to polluted communities.
"After the release of six foreign oil workers under our control two weeks ago, the federal government of Nigeria has since not been ready to negotiate with MEND," the caller said, referring to six of Mr Oswald's colleagues.
"Rather, they have been using cheap blackmail against the people of the Niger Delta and have continued to plan military attacks on defenceless Ijaw people (and) communities," he alleged.
"We wish to reiterate that the federal government and multinationals must meet the demands of the Niger Delta before the remaining three foreign oil workers can be freed," he added.
"Any blip now could be disastrous, as we are battle ready," he warned.
The commander then passed the telephone to a softly spoken young man with a southern American accent who identified himself as Mr Oswald.
"I want to let you know that I haven't seen the other two guys for two days. You need to meet these people's demands real fast.... You need to do what they want and soon and fast," he said.
The hostage said he was still in the location where the hostages had previously been held, but that his US and British colleagues had been moved.
Mr Oswald is one of three oil workers, along with fellow American Russell Spell and British security expert John Hudspith, who have been held in the swamps of southern Nigeria since armed guerrillas seized them on February 18.
The call appeared to have been timed to undermine claims by Nigerian officials, who had earlier confidently predicted that the hostages would be released within days following political talks in the delta city of Warri.
Abel Oshevire, a spokesman for the Delta State government, said Ijaw youth leaders had endorsed an earlier call by tribal elders for the three to be handed over in exchange for talks with the government on local development.
Asked when the release might come, Mr Oshevire said: "No one knows the time.
It could be today, it could be tomorrow. We had to keep two days of vigil the last time six of the nine hostages were released."
Heavily armed militants belonging to MEND attacked energy giant Shell's Forcados export terminal on February 18.
They fought a gun battle with navy troopers, set fire to a crude oil loading platform and kidnapped nine foreign workers, six of whom were later released.
Following the attack, Shell suspended loading at Forcados and evacuated its EA offshore field, cutting oil production in Africa's biggest crude exporter by 455,000 barrels per day, or 20 percent.
Although Nigeria has earned more than 300 billion dollars from oil over the past four decades, the vast majority of its 130-million-strong population still lives in abject poverty and resentment against oil companies runs high.