The strongest hurricane in the Atlantic since Felix in 2007, Matthew now threatens Jamaica, Cuba, Haiti and the Bahamas.
Hurricane Matthew became a powerful Category 5 storm in the southern Carribean on Friday and is expected to hit Jamaica and Haiti late Sunday, the US National Hurricane Center (NHC) said.
The storm is packing winds of 240 kilometres (160 miles) per hour and is the most powerful storm since Hurricane Felix in 2007; the NHC said in its 03:00 GMT bulletin.
The centre of Matthew was located 80 miles (125 kilometres) north-west of Punta Gallinas, Colombia, and 440 miles south-east of Kingston Jamaica.
Authorities in Jamaica and southern Haiti are preparing for heavy rainfall and flooding as Matthew is headed in their direction and forecasters said the region could be hit with 25-38 centimetres (10-15 inches) of rain.
"These rains may produce life-threatening flash flooding and mudslides," the NHC advised.
"Some fluctuations in intensity are possible this weekend, but Matthew is expected to remain a powerful hurricane through Sunday," the NHC added.
Haitian authorities said residents on the southern islands of the country are "first at risk" and the government was making preparations for the storm.
"We invite them to secure the area surrounding their homes and begin to stock up on water and food," Edgar Celestin, a spokesman for the Haitian civil protection agency, told AFP.
The majority of rainfall and wind gusts are expected to come Saturday and Sunday, Haitian officials said.
Matthew became a Category 1 hurricane on Thursday afternoon, then quickly rose to Category 4 the next day.
Ocean swells with the potential to cause dangerous currents and riptides are also possible over the next two days in coastal regions of Puerto Rico, Venezuela, Aruba, Bonaire, Curacao, Colombia, the Dominican Republic, and Haiti.
Hurricane Felix killed some 150 people and left thousands homeless when it slammed the Caribbean coast of Nicaragua in September 2007.