An oily substance of unknown origin is washing ashore in parts of Louisiana that were among the hardest hit by BP PLC's Deepwater Horizon oil spill last year, the U.S. Coast Guard and local government officials said Monday.

The Coast Guard and a Jefferson Parish spokeswoman said they have mobilized oil-spill-response equipment, and the Coast Guard has hired a contractor to lay containment boom in hopes of stopping the substance from penetrating inland waters and ecologically sensitive shorelines.

"We're...