Authorities are trying to determine the scale of the devastation in Jamaica, as Hurricane Melissa slammed into Cuba on its path of destruction.

Most of Jamaica remains in the dark, with 77 per cent of the country without power, after one of the strongest Atlantic hurricanes on record made landfall on Wednesday (AEDT).

Communications are also down across the island. Authorities reported seven people dead in Jamaica and 25 killed in Haiti.

After smashing Jamaica’s south-west as a category five storm, Melissa lost strength and was on Thursday morning (AEDT) grinding across Cuba as a category two.

Melissa roared ashore in Jamaica on Wednesday (AEDT) with top sustained winds of 295 km/h.

A tree fell on a baby in the island’s west, Abka Fitz-Henley, a state minister, told Nationwide News Network, a local radio station.

Emergency services are trying to access the south-west and north-west of the island nation, where most of the destruction was concentrated.

In nearby Haiti, flooding from Hurricane Melissa killed 25 people, officials said.

Jean Bertrand Subrème, mayor of the southern Haitian coastal town of Petit-Goâve, told the Associated Press that 25 people died after La Digue river burst its banks and flooded nearby homes.

Dozens of homes collapsed and people were still trapped under rubble as of Wednesday morning, he said.

Melissa had top sustained winds of 165 km/h and was moving north-north-east at 22 km/h on Wednesday, according to the US National Hurricane Centre in Miami.

The hurricane was centred 70 kilometres north-west of Guantánamo, Cuba, and 335 kilometres south of the central Bahamas.

Preparations for the storm in the Bahamas “should be rushed to completion”, the agency said.

Hundreds of thousands of people in Cuba had relocated to shelters.

A hurricane warning was in effect for Granma, Santiago de Cuba, Guantánamo, Holguin and Las Tunas provinces, as well as the southeastern and central Bahamas.

Melissa was forecast to continue weakening as it crossed Cuba but remain strong as it moves across the south-eastern or central Bahamas later on Wednesday.

By late Thursday, it is expected to be near or to the west of Bermuda.

Haiti and the Dominican Republic also braced for its effects.

The storm was expected to generate a surge of up to 3.6 metres in the region and drop more than 500 millimetres of rain in parts of eastern Cuba.

Intense rain could cause life-threatening flooding with numerous landslides, US forecasters said.

The hurricane could worsen Cuba’s severe economic crisis, which has already led to prolonged power blackouts, as well as fuel and food shortages.

“There will be a lot of work to do. We know there will be a lot of damage,” Cuban President Miguel Díaz-Canel said in a televised address.

He urged the population not to underestimate the power of Melissa – “the strongest ever to hit national territory”.

Jamaican officials reported complications in assessing the damage.

“There’s a total communication blackout on that side,” Richard Thompson, acting director general of Jamaica’s Office of Disaster Preparedness and Emergency Management, told the Nationwide News Network.

More than half a million customers were without power late on Tuesday as officials reported downed trees, power lines and extensive flooding across most of the island.

Extensive damage was reported in parts of Clarendon in the south and in the south-western parish of St Elizabeth, which was “under water”, said Desmond McKenzie, deputy chairman of Jamaica’s Disaster Risk Management Council.

The storm damaged four hospitals and left one without power, forcing officials to relocate 75 patients, McKenzie said.

Britain said on Wednesday it was sending £2.5 million ($5 million) in emergency humanitarian funding to assist the Caribbean region’s recovery from Hurricane Melissa, with targeted support for Jamaica.

The aid package included shelter kits, water filters and blankets to help prevent injury and disease outbreaks, the government said.