The United States coastguard says it has received reports of oil from a huge spill in the Gulf of Mexico washing ashore in the southern state of Louisiana.
A local politician says oil has reached an island near the Mississippi River delta.
Up to 5,000 barrels of oil a day are thought to be leaking into the Gulf of Mexico after an oil rig exploded and sank last week. The slick is 72km by 169km - almost the size of Jamaica.
Efforts to shut down the ruptured well have so far failed, with a third leak now discovered 1,525m beneath the sea.
The coastguard has set fire to part of the spill and is using oil dispersants. The US military is set to join containment efforts.
An emergency shrimping season was opened on Thursday to allow fishermen to bring in their catch before it was fouled by the advancing oil.
The Louisiana coastline, with its rich shrimp and oyster beds, is the most threatened by the spill.
The governor of Louisiana has declared a state of emergency, while the Department of Homeland Security has designated the spill as being of national significance.
The leak began after the Deepwater Horizon oil rig sank on 22 April, two days after it exploded and caught fire while finishing a well for BP.
Under US law, BP must meet all the costs of the clean-up operation.
A Canadian oil well firefighting company, Safety Boss, says it could take months to seal the leaks.