The group kidnapped four foreign oil workers for 19 days last month during a campaign of sabotage that crippled a tenth of Nigeria's oil output and pushed world oil prices to a four-month high. They had demanded the release of the two Ijaw leaders and local control over the delta's oil wealth.
In an email sent to Reuters on Thursday, the Movement for the Emancipation of the Niger Delta (MEND) said: "We believe the Nigerian government intends to poison these individuals and [we] warn of terrible consequences should this plan materialise.
"We will resume our attacks on oil installations of our choosing with effect from the second week in February."
MEND is also demanding $1.5 billion in pollution compensation to villages from Royal Dutch Shell.
The jailed Ijaw leaders are Diepreye Alamieyeseigha, the former governor of Bayelsa state, and Mujahid Dokubo-Asari, a militia leader.
Alamieyeseigha was charged with money laundering in December and is on trial for corruption.
Mujahid Dokubo-Asari is one of
the group's jailed leaders
He was taken to hospital at the weekend for an unexplained illness, prosecutors said.
Asari, who faces treason charges, was removed from prison in Abuja on Wednesday and taken to an undisclosed location for his own safety, police said.
The militants waged a six-week campaign of violence against the oil industry during which Royal Dutch Shell to cut 221,000 barrels a day of output.
The company later resumed production at its 115,000 barrels-per-day EA oilfield where the abduction took place. The rest is expected back shortly, sources said.
The militants warned Shell not to repair its damaged pipelines and oil platforms and said contractors caught at previously attacked installations would be killed.
"We will attack these companies and facilities with unimaginable ferocity as we have no intention of taking hostages in future attacks," the group said.
"We will attack these companies and facilities with unimaginable ferocity as we have no intention of taking hostages in future attacks"
Statement by the Movement for the Emancipation of the Niger Delta
The militants had warned foreigners last month to leave the delta, which pumps most of Nigeria's 2.4 million barrels of oil a day, and said they aimed to cut exports by 30% this month.
The hostages - an American, Briton, Bulgarian and Honduran - were released on 30 January, but the government has not disclosed the terms of the deal.
One militant source said 100 million naira ($770,000) was paid to the kidnappers, but this was denied by the militants themselves.
Olusegun Obasanjo, the Nigerian president, had denounced the kidnappers as "rascals who are practising the things they watch on television", but diplomats fear that they may represent a new, more dangerous threat to the oil industry.
Poor villagers feel cheated out of
their resources by oil firms
Oil industry kidnappings and sabotage are frequent in the Niger Delta, where poor fishing villages are reluctant hosts to a multi-billion dollar industry.
Many in the region feel cheated out of their resources by the oil firms and central government, and some of the militants' demands have been echoed by local politicians.
Industry officials estimate that about 100,000 barrels a day of crude oil is stolen from pipelines in the delta by criminal syndicates working with international smuggling rings.
Much of the money is spent on arms, fuelling a cycle of violence.