GALVESTON, TEX., JUNE 17 -- Shifting winds pushed an oil slick from the Norwegian supertanker Mega Borg northeastward today, and Coast Guard officials alerted communities north of Galveston to Louisiana that tar balls could soon be washing up on their shores.

Coast Guard officials late tonight predicted tar balls would hit land Tuesday. "It's moving slow. It didn't move any significant amount today," Petty Officer David Oney said.

The 30-mile slick, which lingered 11 miles offshore, has stretched into a light sheen in places and continued to drift slowly away from the tanker, crippled 57 miles offshore from Galveston since a June 8 explosion. But officials said that the leaking had apparently stopped and the threat of a new fire had ebbed.

The Coast Guard predicted that the tar balls could begin showing up in a 45-mile stretch of coastline from High Island, about 30 miles north of Galveston, to Sabine Pass at the Texas-Louisiana border.

Salvage crews working on the tanker began transferring oil from a leaking cargo tank to another holding tank aboard the vessel, cutting off the flow of oil into the water.

Meanwhile, officials launched another experiment using oil-eating microbes spread over part of the slick in the first open-sea test of whether the bacteria would harmlessly devour the slick.

"The ship has definitely quit leaking," said Coast Guard Petty Officer Mark Sedwick. "A reconnaissance crew flew over . . . and didn't find any oil within five miles of the ship. That's a pretty good indication that they've stopped the leak."

He said crews might begin moving the remainder of the Mega Borg's cargo to smaller ships on Monday.