Bertha, the first Atlantic hurricane of the season, weakened into a Category 1 storm as it slowly moved toward Bermuda, U.S. forecasters said Thursday.

As of 11 a.m. ET, the storm was around 780 kilometres southeast of the island and its top sustained winds decreased to 150 km/h, the National Hurricane Center said.

The agency warned it was still too early to determine the extent to which the storm could impact Bermuda. Large swells began striking the island and are expected to persist for the next few days, it said.

The Atlantic hurricane season runs from the beginning of June until the end of November.