Nigeria’s navy has joined the search for the gunmen off the Niger Delta. Two navy sailors, among a group providing security, were wounded in the raid.
It is not yet clear which country or countries the foreigners are from.
In 2009 an amnesty for thousands of militants brought peace to the Niger Delta after years of violence.
The ship that was attacked belongs to Sea Trucks Group, based in Rotterdam.
The heavily polluted delta is Nigeria’s main oil-producing region. Oil output has risen since the amnesty took effect, but critics say the peace deal remains precarious.
The militants targeted foreign oil companies, drawing on local support in a region still blighted by poverty despite huge profits for the multinational energy corporations.
Local people wanted to see more of that wealth used to improve their housing and services.