Eight foreign oil workers have been kidnapped by armed men from an offshore oil rig in southern Nigeria.
Numerous armed militia groups operate in the Niger Delta
Six Britons, a Canadian and a US national were seized in the Niger Delta, say police and the rig's owners. No-one has admitted responsibility.
The rig reportedly sent out a distress call, saying it was under attack from between 20 and 30 men in speedboats.
There has been a spate of recent attacks in the region by militants, who want more local control of oil wealth.
They have kidnapped oil workers and warned them to leave the Delta.
The upsurge of attacks on foreign oil interests has cut Nigeria's oil production by 25% - a key factor in the high world price of crude oil.
The BBC's Alex Last in Lagos says this attack on a rig 20km out to sea will worry Nigeria's oil industry, which had thought its offshore facilities were relatively safe.
"Some unknown persons boarded the rig at 0300 [local time, 0200 GMT] and took eight workers," an executive from one of the companies that operate the Bulford Dolphin rig told Reuters news agency.
The attackers in four boats fired shots into the air before boarding the rig, security sources told our correspondent.
The rig, which is owned by the Norwegian company Fred Olsen Energy ASA, lies off the coast, near the town of Warri.
The company says that drilling at the rig has been suspended.
There were 84 oil workers on the rig when it was attacked, said a spokeswoman for Aberdeen-based Dolphin Drilling, which operates the rig.
There were plans to evacuate some non-essential personnel to the mainland, while the crew on board could phone their families.
She said the kidnappers were seeking to negotiate with the companies who contracted the firm.
Nigeria's President Olusegun Obasanjo is trying to secure the release of the men, his spokeswoman told the AFP news agency.
"The kidnap is as a result of a problem between the host community and the oil company," said presidential spokeswoman Remi Oyo without giving details.
A British Foreign and Commonwealth Office spokeswoman said: "We are in touch with both the Nigerian authorities and the men's company and are making urgent efforts to find out more information."
Poverty and pollution
The Niger Delta is home to Nigeria's oil industry, but there is widespread poverty and numerous armed militia groups which operate in the area.
One group, the Movement for the Emancipation of the Niger Delta (Mend), says it is fighting for greater local control of the oil revenues and compensation from oil companies for pollution in the Delta.
Our correspondent says in most previous cases armed groups have taken hostages to extort money from oil companies and the government.
In recent months, Mend has twice taken foreigners hostage in a series of raids and in both cases the men were eventually released unharmed, our correspondent says.
In April, Mend rejected President Olusegun Obasanjo's offer of thousands more jobs and a new motorway for the area, saying it did not address its demands for more local control of oil wealth and demilitarisation.
Since then the group has claimed responsibility for two car bomb attacks, one of which it said was a warning against Chinese expansion in the region.