The militants have previously co-operated with the state government
Nigerian oil militants say they have kidnapped at least 15 foreign hostages in the restive Niger Delta region.
The kidnapping follows a clash between militants and soldiers in Delta state on Wednesday.
A military spokesman confirmed two vessels had been hijacked overnight and the crews taken hostage, but could not give any more details.
The clashes could spark a new wave of violence in Nigeria's southern oil-producing swamps.
It is not immediately clear exactly how many hostages have been taken.
A militant source told the BBC they had seized "about 20" hostages but could not confirm their nationality.
"The hostages will be used as human shields," the militant source said. Lt Col Rabe Abubakar said the militants took a cargo ship and its crew, along with another ship loaded with petrol belonging to the Nigerian National Petroleum Corporation.
Government Tompolo, the militant leader in the area of the attacks and kidnappings, had been co-operating with the state government's request for a ceasefire for a year, analysts say.
Now the Movement for the Emancipation of the Niger Delta (Mend) has ordered foreign oil workers to leave the delta by midnight on Friday.
They have warned they will shoot down any helicopters used by oil companies to ferry expats to oil installations.
The renewed violence between militants and the military's Joint Task Force was expected by analysts after Mend rejected an amnesty offered by the government.
The government has still not officially said the offer is off the table.
Mend is still holding British hostage Matthew Maguire who was seized from an oil services boat in September last year. His colleague Robin Hughes was freed in April.
Militant groups in the Niger Delta have flourished amid a lack of governance and rule of law.
They fund their activities with oil theft, extortion and kidnapping.
The Joint Task Force, charged with bringing security to the Delta, is accused of brutality and corruption.