(Adds details) PORT HARCOURT, Nigeria, April 23 (Reuters) - At least four people were killed in clashes between armed rival criminal gangs in Nigeria's oil city of Port Harcourt, police and witnesses said on Monday. Two people were killed late on Sunday when one gang attacked a rival group in the Borikiri district of Port Harcourt, police said. Witnesses said two other people were killed in a reprisal attack in the Sangana district of the city on Monday and that sporadic gunshots could be heard from hundreds of metres (yards) away. Businesses round Nnamdi Azikiwe Road, epicentre of the fighting, were all forced to close until the gangs were dislodged by troops deployed to the area. Gang clashes are a common occurrence in Port Harcourt, the main city in Nigeria's oil heartland, the Niger Delta. Gunmen frequently engage in turf wars to assert their supremacy. There is also an uneasy standoff between troops and militants in the vast maze of mangrove-lined creeks. The area accounts for all oil output from Nigeria, the world's eighth-biggest exporter.
