Forecasters say tropical storm Ernesto has formed in the Atlantic and is speeding toward a chain of small, popular vacation islands in the Caribbean Sea.

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The National Hurricane Center in Miami said tropical storm Ernesto was headed west towards the Caribbean. (National Hurricane Center)

The National Hurricane Center in Miami said Thursday afternoon that the storm is headed west at about 35 km/h toward the Caribbean. Tropical storm warnings are in effect for the islands of Barbados, Dominica, St. Lucia, St. Vincent, Martinique, Guadeloupe and the Grenadines.

The storm's maximum sustained winds are rapidly gaining strength and were at 80 km/h Thursday evening.

Forecasters say the storm is expected to strengthen further. Its general track takes it toward Jamaica, though forecasters warn that the path of a tropical system can be unpredictable. Cuba and some countries in Central America also could be affected.