An oil spill has been detected along the coast of southern Nigeria near operations for US giant ExxonMobil, the company said Monday, but the cause and size of the leak remained unclear.

Exxon local subsidiary Mobil Producing Nigeria confirmed that oil had been found along the shoreline of Akwa Ibom state in southern Nigeria, with residents also reporting seeing oil deposits along the coast.

A statement said the company "confirms that oiling from an unknown source has been sighted along the shoreline near Ibeno, Akwa Ibom state."

An emergency response team has been sent to the shoreline and a sample has been collected "to determine its source, which remains unknown," the statement said. Government agencies have been notified, it added.

A regional director of the state-run National Oil Spill Detection and Response Agency, Irvin Obot, confirmed the spill. He said his agency could not yet estimate the quantity.

"Investigations are already under way to determine the source and cause of the spill," Obot said.

Community residents and fishermen reported sighting oil along the Atlantic coastline in Ibeno, an oil community where Exxon has facilities. Exxon also has offshore operations in the area.

The village head of Ibeno's Atia community, Obong Ukott Esenem, said fishermen noticed the spill.

"Our community has witnessed another oil spill which has taken its negative toll on the environment," he told AFP.

Nigeria is Africa's largest oil producer. Its oil-producing Niger Delta region has been contaminated by decades of pollution.

Spills occur frequently in the region due to a range of factors, including sabotage by oil thieves or militants as well as operational or maintenance faults by the oil companies.