Hurricane Matthew has weakened slightly as it moves towards Jamaica, but is still packing winds of up to 250km/h (155mph), strong enough to wreck houses, forecasters say.
It is now a category four storm, the US National Hurricane Center says, after reaching the top category five on a scale of intensity.
An urgent meeting of Jamaica's parliament will discuss preparations.
The storm is expected to make landfall by Monday.
Jamaica's palm-lined southern coast is expected to be hit first. The capital, Kingston, is located in the area, as is the country's only oil refinery.
Officials have warned the high winds could also batter the island's main tourist areas including Montego Bay in the north.
"The government is on high alert," the prime minister's director of communications was quoted as saying by Reuters.
"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," Robert Morgan said.
Local emergency teams as well as the police and army are on standby, while shelters are being set up throughout the island, Mr Morgan said.
As the storm approaches many Jamaicans are stocking up on water and food.
Tropical storm warnings have also been issued for parts of coastal Colombia and Haiti over the weekend.
Haitian authorities say the priority is to protect the southern islands of the country, whose inhabitants they have described as "first at risk", according to AFP news agency.
Forecasters said up to 38cm (15 ins) of rain could fall across Jamaica and on southern Haiti.
While Jamaica was damaged by Hurricane Gilbert in 1988, the last major storm in the region was Hurricane Sandy in 2012.
Matthew could be the most powerful storm to hit the island since records began, meteorologist Eric Holthaus.
It is now a category four storm, the US National Hurricane Center says, after reaching the top category five on a scale of intensity.
An urgent meeting of Jamaica's parliament will discuss preparations.
The storm is expected to make landfall by Monday.
Jamaica's palm-lined southern coast is expected to be hit first. The capital, Kingston, is located in the area, as is the country's only oil refinery.
Officials have warned the high winds could also batter the island's main tourist areas including Montego Bay in the north.
"The government is on high alert," the prime minister's director of communications was quoted as saying by Reuters.
"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," Robert Morgan said.
Local emergency teams as well as the police and army are on standby, while shelters are being set up throughout the island, Mr Morgan said.
As the storm approaches many Jamaicans are stocking up on water and food.
Tropical storm warnings have also been issued for parts of coastal Colombia and Haiti over the weekend.
Haitian authorities say the priority is to protect the southern islands of the country, whose inhabitants they have described as "first at risk", according to AFP news agency.
Forecasters said up to 38cm (15 ins) of rain could fall across Jamaica and on southern Haiti.
While Jamaica was damaged by Hurricane Gilbert in 1988, the last major storm in the region was Hurricane Sandy in 2012.
Matthew could be the most powerful storm to hit the island since records began, meteorologist Eric Holthaus.