Attacks by Mend and other groups have severely cut Nigeria's oil output
Militants in Nigeria are holding six members of the crew of an oil tanker, including its Russian captain.
A statement from the managers of the ship, the Sichem Peace, said armed men boarded the vessel near Escravos and forced the men to come ashore.
The ship was released and the remaining 13 crew moved it further offshore.
A statement from the Movement for the Emancipation of the Niger Delta (Mend) said it was holding the men - three Russians, two Filipinos and an Indian.
It said it had kidnapped the crew because they had disregarded its earlier warning to keep away from the Niger Delta.
"Their arrest is meant to serve as a warning to others that there are root issues that have to be resolved with the Nigerian government before normalcy can resume," the group said.
This is the latest in a series of violent incidents in the Niger Delta, which has sharply cut Nigeria's oil production.
The Sichem Peace, which was seized about 20 nautical miles (37 km) from Escravos, was built in 2005 and sails under the flag of Singapore.
Its managers, EMS Ship Management, said it was "working with the relevant authorities and professional advisors, and doing everything possible to secure the timely and safe release of the crew members."
It added that the safety of the crew was its "absolute priority".
The ship, it said, was now 100 miles offshore, and under the control of the remaining crew.
Escravos is one of Nigeria's main oil complexes, including a gas-to-liquids project and export terminal.
Earlier, Mend said it had blown up a major oil pipeline belonging to US firm Chevron, and an oil well head operated by Royal Dutch Shell.
The statement has not been verified and there has been no response from Chevron.
Last month Nigerian President Umaru Yar'Adua offered a 60-day amnesty to the militants of the Niger Delta, if they gave up their struggle.
But Mend says it will not disarm until the government releases one of its leaders, Henry Okah.
Mend says it wants the people of the Niger Delta to derive greater benefits from the region's oil.
But many criminal gangs have taken advantage of the lawlessness in the area.