Police say gunmen have shot dead four police officers in Nigeria's oil-rich southern delta.
The shootings follow a period of relative calm in the region, where a government-sponsored amnesty has appeased militants who once crippled the oil industry by targeting pipelines and expatriate workers.
Police spokesman Eguavoen Emokpae says the officers killed Thursday were on a boat patroling the Nembe river in Bayelsa State. It was not immediately clear if the gunmen shot from land or also were on boats.
Nigeria is Africa's biggest oil producer and a top exporter to the U.S.
Foreign firms have pumped oil for more than 50 years out of Nigeria's southern delta, but many Nigerians there believe their communities do not get the financial benefit they deserve from the industry.