Tropical Storm Chantal on Tuesday churned toward Puerto Rico, the Dominican Republic and Haiti, unleashing driving rains and winds near hurricane strength.

At 2100 GMT, Chantal was about 270 miles (440 km) southeast of Puerto Rico, packing top sustained winds of 65 mph (100 kmh), the Miami-based National Hurricane Center said.

The storm was grinding west-northwest at 26 mph (43 kmh), and tropical storm conditions were expected in Puerto Rico, a densely populated US territory, later Tuesday.

Haiti, the poorest country in the Americas, is in Chantal's crosshairs as it is due to dump heavy rain on mountainous Hispaniola, the island it shares with the Dominican Republic to the east.

Bahamian authorities issued a tropical storm watch, and a warning -- which means storm conditions are expected in a matter of hours -- was in effect for the entire coast of Haiti, most of the Dominican Republic, Puerto Rico, the Turks and Caicos and the southeastern Bahamas.

Storm surges of up to a meter or more are expected in mountainous Puerto Rico, with over four million people; the Dominican Republic to the west, with 10 million; and further west Haiti, with 10 million people.

Heavy rain and tropical storms can be deadly in rainy season. Heavy rains pick up speed and force on slopes and can wash away riverside communities. It is a major concern in the region, especially in Haiti where soil erosion is a problem.

Haiti is struggling to recover from a January 2010 earthquake that killed 250,000 people.