Six Britons, one American and one Canadian were seized from an oil rig 65km off the coast in a raid on Friday by 20 to 30 attackers in speedboats.

Two Britons were freed in the early hours of Sunday and the rest several hours later, the embassy spokesman said.

"I understand they're all in good health," he said.

The authorities declined to comment on whether a ransom had been paid, but a security source close to the situation said money had changed hands.

Nigeria, which normally pumps 2.5 million barrels of crude a day, is the fifth-largest source of oil imports to the US.

The attack had no impact on oil output as the facility is an exploration rig that will not produce crude for years.