The first tropical storm of the 2012 Atlantic hurricane season formed Saturday off the southeastern US coast with winds of 45 miles (75 kilometers) per hour, US weather forecasters said.

Dubbed "Alberto," the storm was 140 miles (225 kilometers) southeast of Charleston, South Carolina, moving southwest at a speed of three miles (six kilometers) per hour, the National Hurricane Center.

It posed no immediate threat to land but a tropical storm watch may be needed for parts of the Carolina coasts Saturday night, the Miami-based center said.

"Maximum sustained winds are near 45 miles (75 kilometers) per hour with higher gusts," it said. "Some strengthening is possible over the next day or so."

Alberto was the earliest tropical storm to form in the Atlantic since 2003, and it was the first time one has formed before the official start of the hurricane season in both the Atlantic and east Pacific basins.

The hurricane season runs from June 1 to November 30, usually peaking in August or September.

The first tropical storm of the season in the Pacific formed Tuesday off the coast of Mexico.