LAGOS: Authorities have confirmed that gunmen disguised as riot police have abducted four foreign workers from the residential compound of oil services giant Schlumberger lt, SLB. Ngt, in Nigeria's oil city Port Harcourt.

Kidnapping has become an almost daily occurrence in the anarchic Niger Delta, home to Africa s largest oil industry, and about 30 foreigners are now being held by various armed groups in the vast wetlands region.

"Some expats were kidnapped from the club of Schlumberger Anadrill in a residential area last night," said Rivers state police commissioner Felix Ogbaudu, adding that the abductors were dressed as riot police.

Schlumberger Anadrill is a private subsidiary of U. S., listed Schlumberger, the world's largest oil services company.

A security source with an oil company in the area said the four hostages were citizens of Britain, France, the Netherlands and Pakistan.

Attacks on oil facilities and kidnappings have forced thousands of foreign workers to flee, and reduced oil output from the world's eighth-largest exporter by almost a million barrels a day, or one third of Nigeria's capacity.

Some armed groups say they are fighting for more autonomy for the impoverished oil province of southern Nigeria, but the line between militancy and crime is blurred and most abductions are by groups seeking ransom.