Both the captain and an engineer were taken from the ship flying a Turkish flag and chartered by French oil giant Total.
So far, the nationalities of the two captives have not been disclosed. The ship, however, was not taken by the gunmen, Nigerian officials said.
The capture of the sailors by unknown gunmen followed the release of another captive, a British man, earlier this week by Nigeria's leading militant group, the Movement for the Emancipation of the Niger Delta.
Robin Barry Hughes was one of 27 oil workers kidnapped in September, officials said.
His captors handed Hughes over to military officials in Nigeria. MEND said earlier in an e-mail that Hughes, 59, was being released "based on the milk of human kindness and compassion" due to his age and health.
However, MEND said another Briton, Matthew John Maguire -- who was kidnapped along with Hughes and the others when an oil supply vessel was hijacked -- would not be released along with Hughes. Most of the other hostages were released shortly after they were taken in September, the report said.
MEND had maintained Hughes and Maguire would be held until the government of Nigeria released MEND leader Henry Okah, who is on trial in an alleged arms-trafficking case.
In January, the captors released a photo of the pair in which they appeared to be tired but not obviously injured.
The continued attacks on oil installations and industry ships follow Nigerian President Umaru Yar'Adua's offer earlier this month of amnesty to the delta's armed groups that lay down their arms.
Yar'Adua shocked many Nigerians when he made the offer to the groups, saying he would grant the gunmen amnesty if they agreed to lay down their arms and partake in talks.
"We will grant amnesty to all those who are ready to lay down their arms," Yar'Adua said. "It will also include rehabilitating and integrating them into the system."
Nigeria's militants contend their ongoing fight against the oil industry is based on the inequitable distribution of the country's oil wealth. Though the delta has produced more than $300 billion in oil over the last few decades, much of the region remains impoverished and underdeveloped.
Nigerian oil output had once reached about 2.5 million barrels per day. According to recent estimates by the country's oil officials, output has dropped to about 1.6 million bpd, a reduction blamed on attacks on oil and gas installations by armed groups and kidnapping of petroleum workers.
A recent report by government and private observers in the delta found that at least 1,000 people had been killed in militant-related violence in 2008. The violence also cost the sector more than $20 billion in lost production, according to the findings.
Critics of MEND and other militant groups in the delta say their cause is bent on nothing but thievery, pointing to their practice of siphoning oil from the country's pipelines and selling it on the international black market.
The president said his security chiefs were working out details of the planned talks, though no definitive plan for engaging militant leaders has been offered.
MEND has denounced the amnesty offer.
In an e-mail from MEND sent to Nigerian news sources, the group said it would not "lay down its arms because of a mere verbal statement from Mr. Yar'Adua" but would consider talks with the government under the auspices of an international mediator.
So far, the offer to MEND has received mixed reviews from hard-line members of the Nigerian government, who favor continuing the current military tactic of dealing with militants. The Nigerian army's Joint Task Force operating in the delta deals exclusively with the handling of the region's militant and gang violence, which has been blamed for the more than 25 percent shortfall in oil production in recent years.
But others have expressed support for the amnesty proposition.
"All things being equal, the recent presidential amnesty to the Niger Delta militants should go a long way in dousing the tension in that region and possibly make the angry youths put down their arms and embrace dialogue," read an editorial Tuesday in the pages of Nigeria's This Day newspaper.