Four foreign oil workers were kidnapped by armed men from a nightclub in Nigeria's southern oil city of Port Harcourt, the fifth abduction in 10 days, witnesses and police said on Monday, according to Reuters. The kidnappers, who wore army uniforms, released volleys of automatic gunfire into the air as they stormed the site in three vehicles late on Sunday, ordered everyone in the crowded club to lie on the floor and singled out white males for abduction. The hostages included two Britons and an Irish national, diplomats said. The German foreign ministry said it was checking if one of its citizens was also missing. "There was serious shooting in the bar and they left taking away some white men," said bar manager Edith Monigha, adding there had been more than seven attackers. "They didn't rob us or ask for anything else, they only wanted the white men." She said she could not tell how many men were abducted in the raid on Goodfellas nightclub shortly before midnight on Sunday but a colleague said he thought there were four. Blood stains were visible on the floor of the club on Monday. Monigha said several people had cut their legs in the scramble to get down. The gunmen abandoned their cars and escaped in a waiting speedboat after security forces engaged them in a fierce gun battle, police said. "The kidnappers ... burnt one of the vehicles they used, maybe to destroy anything that could give them out, and escaped through the waterway," said a police spokeswoman in Port Harcourt, capital of Rivers state. News of the kidnappings came just after the release of three Filipino gas workers who had been taken hostage 10 days ago. Later on Monday, two other oil worker hostages, a Moroccan and a Belgian who were kidnapped on Aug. 10, were released, a witness said. The wave of kidnappings coincides with an upsurge in militant attacks against the oil industry which has cut oil production by 25 percent in the world's eighth largest exporter 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 principally by ransom, but some recent incidents have taken on a more political tone, with demands reflecting a growing ethnic nationalism among the Ijaw 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. Police named one of those seized on Sunday as Briton John Guyan and said he worked for Smith International Inc., a supplier to the oil and gas industry. The company confirmed one of its expatriate workers had been taken hostage. Another of those kidnapped was Brayan Fogerty, who works for U.S. oil services company Halliburton, police said. Separately on Monday, Norway's ambassador in Nigeria said negotiators were close to a deal to free four other foreign workers -- two Norwegians and two Ukrainians -- kidnapped from an oil services ship off the coast of Nigeria last week.