About ten foreign oil workers are still in captivity, after five were set free in two separate releases on Monday (August 14). The United States embassy in Nigeria said on Tuesday (August 15) that it had received reports that an American citizen was among oil workers kidnapped in the southern oil city of Port Harcourt. At least five foreign workers -- two Britons, a German, an Irish and Polish -- were snatched by armed men from the Goodfellas nightclub shortly before midnight on Sunday (August 13), but authorities still do not have a full list of those abducted. Diplomats said the American may have been abducted in a separate incident in the same city, but no further details were mediately available. The British High Commission in Nigeria said two Britons were among the hostages. Irish public broadcaster RTE reported that Brian Fogarty, an Irish citizen, was also abducted, while the German Foreign Ministry said one of its nationals was taken. It was the fifth kidnapping of oil workers in Africa's top oil producer this month, and follows a wave of militant attacks against the oil industry which have cut Nigerian production by a quarter since February. Militancy is fuelled by widespread feelings of injustice in the Niger Delta region where most people live in poverty despite the wealth being pumped from their ancestral lands. Many abductions are motivated mainly by ransom, but some recent incidents have taken on a more political tone, with demands reflecting a growing ethnic nationalism among the Ljaw tribe, which is native to the Niger Delta. Criminal gangs, sometimes involved in the large-scale theft of crude oil from pipelines, also regularly indulge in kidnapping and extortion, and it is often difficult to distinguish between the two.