The SP Brussels is said to have been boarded by "heavily armed pirates" on Monday, about 65km (40 miles) off the the coast of the oil-producing Niger Delta region.

On the same day, gunmen kidnapped six employees of a South Korean firm.

Kidnappings are common in the Niger Delta, Africa's main oil region.

Following the attack on Monday, the remaining crew sailed the ship to Nigeria's economic capital Lagos, operator Medallion Marine said.

"Medallion Marine have been working in close co-operation with the authorities, and everything possible is being done to ensure the safe return of those crew members taken from the vessel," said the company in a statement. "Nigerian naval vessels are assisting with this process."
Lucrative enterprise

Four foreigners and two Nigerians working for the South Korean company Hyundai were kidnapped on the Atlantic coast of the oil-rich Bayelsa state on Monday, according to the police.

Kidnapping is a lucrative criminal enterprise worth millions of dollars a year in Nigeria. The country has one of the world's worst kidnapping records.

Oil workers and other foreign nationals working in the southern region are often targeted because companies will pay high ransom money to secure their employees' release.

Earlier this month, the mother of Nigeria's Finance Minister Ngozi Okonjo-Iweala was snatched from her home in the Delta state.

She was freed after for five days. The minister said the kidnappers had demanded her resignation.