The helicopter, operated by a private firm and hired by the Nigerian National Petroleum Company Limited, lost contact while flying from Port Harcourt, the Rivers state capital, to an oil production and storage facility, said Olufemi Soneye, the spokesman for the national oil company, in a statement on Thursday.

Aviation authorities said no emergency locator transmitter signal was received from the helicopter while confirming that manual efforts to plot the location of the accident were underway.

Nigeria's aviation ministry said in a separate statement that all available resources, including the military and low-flying aircraft, had been deployed to assist in locating and rescuing survivors, Xinhua news agency reported.

"Search and rescue operations are ongoing with the support of the Nigerian Search and Rescue Unit, the Nigerian Civil Aviation Authority, the National Safety Investigation Bureau, and other relevant agencies," the statement said.