Hurricane Bill has strengthened to a Category 4 storm as it rumbles across the Atlantic with maximum sustained winds near 135 mph.

Forecasters say some additional strengthening is expected by the first hurricane of the Atlantic season.

Bill is centered about 460 miles east of the Leeward Islands and moving west-northwest near 16 mph.

The National Hurricane Center in Miami says people in the Leeward Islands should monitor Bill's progress, though the core of the dangerous storm is expected to pass well to the northeast of the islands late Wednesday and early Thursday.