Streets across Dominica turned into fast-flowing rivers that swept up cars as Tropical Storm Erika pummelled the eastern Caribbean island, unleashing landslides and killing at least four people.

The storm, which forecasters said could reach Florida as a hurricane on Monday, knocked out power and water supplies on Dominica as it dumped 38 centimetres of rain on the small island and headed west into the Caribbean Sea.

An elderly blind man and two children were killed when a mudslide crashed into their home in the southeast of the island, said Police Superintendent Daniel Carbon. Another man was found dead near his home in the capital of Roseau after a mudslide, but the cause of death was could not be immediately determined, Carbon told The Associated Press.

Police said another 20 people have been reported missing.

Erika was centered about 280 kilometres west of Guadeloupe, and was moving west at 24 kph with maximum sustained winds that had slipped slightly to 75 kph, according to the U.S. National Hurricane Center in Miami.

Erika was expected to move near Puerto Rico and the Virgin Islands on Thursday and be near or just north of the Dominican Republic on Friday as it heads toward Florida early next week, possibly as a hurricane.

Chris Landsea, a meteorologist at the hurricane center, said the storm could dissipate if it passed over Hispaniola or Puerto Rico or it could gather and pose a potential threat to Florida next week.

"The uncertainties are very high," he said.

As the storm entered the Caribbean, it did the heaviest damage to Dominica, an island of about 72,000 people of lush forests and steep terrain. Authorities were still conducting a full damage assessment after rivers surged over their banks and walls of mud surged into homes.

Trees, lightpoles strewn across streets

About 80 per cent of the island was without electricity, and water supply was cut off, authorities said. Trees and light poles were strewn across streets as water rushed over parked cars and ripped the scaffolding off some buildings. The main airport was closed due to flooding, with water rushing over at least one small plane.

The main river that cuts through the capital overflowed its banks and surging water crashed into the principal bridge that leads into Roseau.

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People shop for water at a supermarket in preparation for Tropical Storm Erika in San Juan, Puerto Rico on Thursday. (Alvin Baez /Reuters)

"The capital city is a wreck," policewoman Teesha Alfred said. "It is a sight to behold. It's a disaster."

Erika was likely to hit the island of Hispaniola, which is shared by Haiti and the Dominican Republic, said chief forecaster James Franklin at the National Hurricane Center.

"That would certainly not be good news for Hispaniola," he said. "They're very vulnerable to flooding. And even if Erika is a weak system that could be very bad there."

Officials shuttered schools, government offices and businesses across the region and warned of flash flooding because of dry conditions caused by the worst drought to hit the Caribbean in recent years. Authorities warned power and water service might be temporarily cut off.

Puerto Rico Gov. Alejandro Garcia Padilla said the storm could bring badly needed rains to the parched U.S. territory.

"We're happy given the dry conditions, but it does highlight the need to be on alert," he said, adding that heavy downpours could lead to flash floods. He activated the National Guard as a precaution.

The heaviest rains were expected to hit Puerto Rico's eastern region, with the storm expected to pass about 50 kilometres north of the island overnight Thursday, said Odalys Martinez, with the National Weather Service in San Juan.

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Tropical Storm Erika is pictured in the Atlantic Ocean northeast of Venezuela in this Aug. 26 NASA handout satellite photo. Erika, the fifth named storm of the Atlantic hurricane season, closed in on the Eastern Caribbean on Wednesday. (NASA/Reuters)

Erika is expected to dump between 10 to 20 centimetres of rain across the region, with up to 31 centimetres in some areas.

Dozens of flights were canceled in the region, and the U.S. Coast Guard closed all ports in Puerto Rico and the U.S. Virgin Islands.

Ignacio now a hurricane

Meanwhile in the Pacific, Ignacio strengthened into a hurricane. The storm's maximum sustained winds increased Thursday morning to 150 kph.

Hurricane Ignacio was centered about 1,825 kilometres east-southeast of Hilo, Hawaii, and was moving west-northwest near 20 kph.

Also in the Pacific, a new tropical storm formed Thursday morning. Tropical Storm Jimena had maximum sustained winds near 75 kph and was expected to strengthen to a hurricane Friday. Jimena was centered about 1,430 kilometres south-southwest of the southern tip of Mexico's Baja California peninsula.