Exxon Mobil Corp. says it has detected an oil spill along the shoreline of Nigeria’s oil-rich southern delta.

Exxon Mobil spokesman Nigel Cookey-Gam said in an email statement Wednesday that the spill was spotted near Ibeno in the southern state of Akwa Ibom where the U.S. oil company’s local subsidiary has its base of operations.

He said an emergency response team has been sent to the shoreline and that the source of the spill is still unknown.

Oil spills remain common in Nigeria’s Niger Delta, more than 50 years after the country saw its first well open.

Nigeria is a top supplier of crude oil to the U.S.