ABUJA (Reuters) - Kidnappers freed three British and one Colombian worker unharmed on Monday, a police official said, six days after they were seized near the oil hub Port Harcourt in the Niger Delta.

"Yes, they were freed this afternoon in Abia state. No one has been arrested, but the police have a suspect they are investigating," said a police official in neighbouring Rivers state, where Port Harcourt is located.

The expatriates, employed by Netco Dietsmann, were working as contractors for Royal Dutch Shell at the Afam power plant. The reason for their release was not immediately clear.

Local media reports this week said the kidnappers had demanded a 300 million naira ransom.

Kidnappings for ransom are common in the oil-producing Niger Delta with hundreds of incidents reported each year. Most victims are released unharmed after a couple of days.

Gunmen on Sunday kidnapped three Nigerians from their shipping vessel while travelling off the Bonny coast in southern Rivers state, two security sources said earlier on Monday.