LAGOS, Nigeria (AP) ― An oil spill near the coast of Nigeria is likely the worst to hit those waters in a decade, a government official said Thursday, as slicks from the Royal Dutch Shell PLC spill approached the country’s southern shoreline.

The slick from Shell’s Bonga field has affected 185 kilometers of ocean near Nigeria’s coast, Peter Idabor, who leads the National Oil Spill Detection and Response Agency, told The Associated Press. Idabor said the slick continued to move toward the shore Thursday night, putting at risk birds, fish and other wildlife in the area.

Shell, the major oil producer in Nigeria, said late Thursday the spill came from a “flexible export line” connecting the offshore field to a waiting tanker. The company published photographs of the spill, showing a telltale rainbow sheen in the ocean, but said it believes that about 50 percent of the leaked oil has already evaporated.

The source of the leak has been plugged and experts from Britain were coming to help with the cleanup, Idabor said. Nigerian Navy ships also had been sent into the area to help control the spill, he said.