MIAMI (AP) — The National Hurricane Center says a tropical depression has formed in the Gulf of Mexico and it is heading toward the oil spill site off the coast of Louisiana.

BP and Coast Guard officials had already decided to stop drilling on a relief well on Tuesday, before forecasters declared the storm off the southwestern part of Florida was a depression.

A tropical storm warning was issued for much of the Gulf coast affected by the oil spill, from Destin, Fla., to Intracoastal City, La.

The center of the storm was located about 375 miles (600 kilometers) southeast of the mouth of the Mississippi River. It was moving northwest about 6 mph (9 kmh) with winds of 35 mph (55 kmh).

It was expected to strengthen slowly and become a tropical storm on Wednesday.