Two foreign construction workers were kidnapped by gunmen on their way to work in Nigeria's southern oil city Port Harcourt on Tuesday, police said.
Rivers State police commissioner Felix Ogbaudu said the two men were American nationals working for local construction firm Pivot, but oil industry security sources said one of the men was British.
German shipping line Baco-Liner said militants who seized one of its cargo ships on its way to Warri port in Delta State on Saturday had taken hostage all 24 Filipino crew members.
Local government officials said earlier that only six Filipinos had been abducted from the ship in a remote river channel.
A security source said of the two construction workers seized on Tuesday: "They were taken by armed men from their car on the way to work and from there straight to the waterside and into the swamps." The source asked not to be named.
A United States embassy spokesman was not available for comment. The British High Commission said it had yet to confirm the men's nationalities.
Thousands of foreign oil workers have left Nigeria in the past year as attacks and kidnappings have multiplied, and some industry executives see the situation in the Niger Delta descending into anarchy as landmark elections approach in April.
Nigeria is the world's eighth largest oil exporter, but militant raids last year have cut oil exports by a fifth.
Baco-Liner Managing Director Klaus Steffen told the agency he had no contact with the crew of the ship his firm operates, and assumed that the ship and all 24 crew members were being held.
"We have had no contact with the crew so I cannot confirm if they are still on board," Steffen said by telephone from Duisburg.
Government officials say the Filipinos' kidnappers have demanded the release of two jailed leaders from the Niger Delta among other demands, and that talks have begun.
Another militant group is still holding three foreign oil workers — two Italians and one Lebanese — after abducting them from an oil export terminal on December 7.
That group is also demanding the release of two jailed delta leaders, on top of compensation for decades of oil pollution to delta villages and regional control over the delta's oil wealth.
The April elections should mark Nigeria's first fully democratic transition and many expect politicians to arm thugs to gain sway at the polls, especially in the delta where elected office comes with a slice of oil revenue.
The delta accounts for all Nigeria's oil production and poverty fuels militancy and crime in its remote communities, where residents complain of neglect and marginalisation.