Torrential rains have lashed eastern Cuba, knocking down trees and forcing thousands to evacuate as Tropical Storm Noel moves towards the Bahamas.
Hurricane warnings have been dropped for the Bahamas, with the storm slowing over Cuba, US forecasters said.
At least 20 people were killed amid floods caused by Noel in the Dominican Republic. Most died on the south coast, east of the capital Santo Domingo.
It is feared the death toll there will rise, with another 20 reported missing.
By 1500 GMT, Noel's centre was about 40 miles (60km) south-east of Camaguey, Cuba, and some 270 miles (435km) south of Nassau in the Bahamas.
The US National Hurricane Center (NHC) in Miami said the storm is expected to remain over Cuba for the next 24 hours.
At least 10in (25cm) of rain has already fallen on the island of Hispaniola - divided between the Dominican Republic and Haiti - and rain is continuing to fall there, forcing river levels higher.
"These rains, particularly in Hispaniola, are expected to cause life-threatening flash floods and mudslides," the NHC said.
Several bridges are reported to have collapsed, cutting off communities, and early on Monday the entire power system for the Dominican Republic was temporarily knocked out.
Hundreds of people were being evacuated, amid fears of flash floods and landslides, the country's authorities said.
Great damage has already been done to crops.
At least 10 people died when a river burst its banks east of Santo Domingo in the single worst incident reported.
Officials closed the international airport in Santo Domingo, and Dominican President Leonel Fernandez held emergency talks to co-ordinate disaster response.
Reports suggested between three and five people died in another incident in San Jose de Ocoa, also to the east of the capital.
In Santo Domingo, schools have been closed, with some being used as shelters, high-school student Richard Pichardo told the BBC.
"We were unaware of the power of the storm until it arrived..." he said.
"Water falls from the sky continually, which never happens over here."
Hispaniola is particularly vulnerable to flooding because of its steep hills and because many houses are no more than shacks.
While there were not yet any reports of fatalities from Haiti, the country often suffers worse from flooding than its neighbour because so much of it has been deforested.
Heavy rain destroyed homes in three Haitian departments, officials said.
The storm had been forecast to hit Haiti hardest, but veered towards the Dominican Republic on Monday.
"I think this has taken some officials by surprise. The storm was predicted to go more toward Haiti," aid worker Holly Inuretta told the Associated Press news agency.
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