A swirl of bad weather developed into Tropical Storm Erika just east of the Caribbean basin near the northern Leeward Islands, the US National Hurricane Center said.
Island governments across the region issued storm watches in preparation for the arrival of Erika, which at 2630 IST was moving west-northwest at a steady 15 kilometres per hour, the Miami-based NHC said.
Maximum sustained winds were almost 85 kilometres per hour, and "some slow strengthening is forecast during the next couple of days," the centre said.
A tropical storm has winds of up to 105 mph (165 kph) before it is classified as hurricane force.
On its current route Erika was expected to remain northeast of the Leeward Islands.
Tropical storm watches were issued for the Netherlands Antilles, Antigua, Barbuda, St. Kitts, Nevis and Anguilla as well as the islands of St. Martin and St. Barthelemy.
The Atlantic hurricane season began on June 1 and ends on November 30. So far, 2009 has seen one of the calmest starts to the hurricane season in a decade, which researchers have attributed to the development of an El Nino effect in the Pacific.
Meanwhile, Hurricane Jimena in the east-central Pacific weakened to a Category 3 storm as it bore down on Mexico's Baja California peninsula.