The storm is expected to remain over open waters and move north-northeast of islands including Antigua and Barbuda, the United States and British Virgin Islands and Puerto Rico, according to the National Hurricane Center in Miami.
Erin was located about 1270km east of the Northern Leeward Islands. It had maximum sustained winds of 95km/h and was moving west at 28km/h.
Tropical storm watches were in effect for Anguilla and Barbuda; St. Martin and St. Barthelemy; Saba and St. Eustatius; and St. Maarten.
Erin is forecast to become a hurricane by Friday and strengthen into a Category 3 storm by late Saturday, which would mark the first major storm this season.
"Erin is moving into an area of the Atlantic primed for rapid intensification. The waters are incredibly warm," said Alex DaSilva, lead hurricane expert for AccuWeather.
Tropical-storm force winds could occur in parts of the northern Leeward Islands, the US and British Virgin Islands and Puerto Rico this weekend, forecasters said.
"There is still a greater than normal uncertainty about what impacts Erin may bring to portions of the Bahamas, the east coast of the United States, and Bermuda in the long range," the hurricane centre said.
Hurricane specialist and storm surge expert Michael Lowry said nearly all models have Erin turning "safely east of the broader US next week."
Erin is the fifth named storm of the Atlantic hurricane season, which runs from June 1 to November 30.
Forecasters are expecting another unusually busy season for the Atlantic, with predictions calling for six to 10 hurricanes, with up to half reaching major status.
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