Militants say they are fighting for a fairer distribution of oil money
Militants from Nigeria's Niger Delta region have attacked a tanker carrying thousands of tonnes of diesel and kidnapped a Romanian crew member.
The unidentified militants blew up the ship's engine with dynamite, security sources said.
The attack happened in the major shipping lane off the Rivers State coast just within the military protection zone for vessels.
The vessel is believed to be still at sea and seriously damaged.
The MT Meredith was carrying 4,000 tonnes of diesel from Lagos to Port Harcourt, security sources said.
The Movement for the Emancipation of the Niger Delta (Mend) said the gunmen were from an "affiliate" group.
"Mend is in touch with the group and will ensure the abducted man is released unharmed at the earliest convenience," the group said in an e-mail to journalists.
At the weekend a tanker hired by Royal Dutch Shell was attacked by militants.
They locked the crew in the hold and ransacked the vessel.
The BBC's Andrew Walker in Nigeria says acts of piracy in the waters off the Niger Delta are common, but attacks on tankers are unusual.
Nigerian militants tend to attack oil services boats closer to the coast, stripping them of equipment and kidnapping the foreign oil workers.
Last year, the Mend, Nigeria's most publically visible group, attacked the Bonga oil facility, an offshore oil field operated by Royal Dutch Shell.
The ship-like oil platform was 75km (46 miles) off the coast and thought to be out of militants' reach.
Some militant groups say they are fighting for a fairer distribution of wealth from Nigeria's oil.
But there are many armed groups in the Delta who make money out of extortion, kidnapping and oil theft under the direction of powerful and well-connected political figures.
The United Kingdom has promised military training to help the government combat the problem.
Oil production has been cut by a fifth in the last three years partly as a result of violence.