LAGOS: Armed pirates who stormed an oil service ship in southern Nigeria have kidnapped six foreigners and demanded a $1.3 million ransom for their release, police told AFP.

"Three of those abducted are from Ukraine, two from India, one from Russia," Bayelsa state police spokesman Fidelis Odunna said of the Sunday attack.

"One of the kidnappers called to demand the sum of 200 million naira" (one million euros, $1.3 million), he added.

The Armada Tuah vessel operated by the Lagos-based Century Group with a crew of 15 was attacked by gunmen in waters off Bayelsa, police said.

It was not immediately clear how far offshore the vessel was at the time of the attack, or whether it had in fact docked.

"We have deployed intelligence personnel in search of the six workers," Odunna said.

The kidnapping of foreign oil workers is common in Nigeria's oil-rich south, with the hostages often released following a ransom payment. It is however rare for police to discuss the details of ransom demands.