Bertha, the first Atlantic hurricane of the season, has weakened to a Category 2 storm in the open sea, U.S. forecasters said.

As of 11 a.m. ET Tuesday, the storm's centre was about 1,000 kilometres east-northeast of the northern Leeward Islands and about 1,500 kilometres southeast of Bermuda.

The storm's maximum sustained winds of 168 kilometres per hour and was headed in a northwest path toward Bermuda at a speed of about 16 km/h. It's unknown if or when the hurricane will make landfall.

The Atlantic hurricane season runs through Nov. 30.