Southeasterly winds had driven the slick just off the Louisiana coast, National Oceanographic and Atmospheric Administration spokesman Charlie Henry said.

Oil company BP, whose ruptured well is at the heart of the spill, and state and federal agencies have strung miles of floating booms around the leading edge of the shoreline in an effort to contain the spill, but authorities said it could begin affecting some areas of the coast by April 29 evening.

Efforts to shut down the well have failed so far, and more complicated plans may take weeks. Louisiana Gov. Bobby Jindal on April 29 declared a state of emergency ahead of the oil slick's arrival, warning that it covered as much as 600 square miles of water.

President Obama is sending Department of Homeland Security Secretary Janet Napolitano, Department of the Interior Secretary Ken Salazar and Environmental Protection Agency Administrator Lisa Jackson to Louisiana on April 30 to inspect the effort to contain the oil slick, his administration announced on April 29.

Ten wildlife refuges in Mississippi and Louisiana are in the oil's likely path, with the Pass-a-Loutre Wildlife Management Area at the tip of the Mississippi River likely to be the first affected, Jindal announced.