One of the most powerful Atlantic hurricanes in years weakened a little on Saturday as it dropped torrential rains on Colombia's coast before moving across the Caribbean on a course towards Jamaica, Haiti and Cuba with potentially devastating winds and rain.

In Colombia there were reports of one death and damage to 18 houses along the La Guajira peninsula. A man was swept to his death by a flash flood in an area where there had been no rain for the last four years.

 The US National Hurricane Center (NHC) downgraded its designation of Hurricane Mathew by one level on Saturday, from the top Category 5 on the Saffir-Simpson scale to Category 4. But it remains the strongest Atlantic hurricane since Felix in 2007.

"Hurricane Matthew could rival or possibly exceed Gilbert if the core of the strongest winds does actually move over Jamaica," Dennis Feltgen, a meteorologist and spokesman for the NHC in Miami said. "There is no certainty of that at this point."

"The government is on high alert," Robert Morgan, director of communications at the Jamaican prime minister's office, said. "We hope that the hurricane does not hit us, but if it does hit us, we are trying our very best to ensure that we are in the best possible place."

As of 1500 UTC on Saturday the storm was centered about 390 miles (625 kilometers) southeast of Kingston. It was moving west at 6 mph (9 kph) and was expected to reach the eastern part of Jamaica on Monday before moving on towards Haiti, Cuba and the Bahamas, with a slight chance of brushing the Florida coast in the US.

Jamaica, Haiti and Cuba under threat

Jamaica and Haiti could see 10-15 inches (25-38 centimeters) of rain, according to forecasters at the NHC. "These rains may produce life-threatening flash flooding and mud slides," the NHC's Feltgen warned, saying isolated areas could be inundated with up to 25 inches of rain. "Some fluctuations in intensity are possible this weekend, but Matthew is expected to remain a powerful hurricane."

Preparations were being made in Haiti: "We will prepare with drinking water for the patients, with medication, with generators for electricity, available vehicles to go look for people at their homes," said Yves Domercant, the head of the public hospital in Les Cayes in the south of Haiti.

Cuba has opened shelters and organized volunteers to clean storm drains and gather food stocks in Santiago de Cuba, the island's second city, in preparation of the hurricane. 

"We have to work intensely," Lazaro Exposito Canto of the Communist Party central committee said in an interview with the "Granma" newspaper. Volunteers would go from house to house to warn of the storm, he said.

bik,jm/rc,jlw (Reuters, AFP, AP, dpa)