Hurricane Melissa's winds dipped to a still "powerful" storm, as it passed over Jamaica, where it landed with historic ferocity and torrential rains.
"Melissa remains a powerful Category 4 hurricane over northwestern Jamaica," the US National Hurricane Center (NHC) said in its latest advisory.
The NHC added that the storm was still packing maximum winds of 240 km/h.
"This is an extremely dangerous and life-threatening situation," it said.
Melissa made landfall near the town of New Hope, some 62km south of Montego Bay, packing maximum sustained winds of 295km/h, the NHC said.
Watch: Hurricane Melissa bears down on Jamaica
The slow-moving storm is forecast to remain a powerful hurricane as it crosses the mountainous island, whose highland communities are vulnerable to landslides and flooding, and heads towards Santiago de Cuba, Cuba's second-largest city.
The Miami-based hurricane centre warned that "total structural failure" was likely in Melissa's path.
"The destruction could be unlike anything people in Jamaica have seen before," said US forecaster AccuWeather's lead hurricane expert, Alex DaSilva.
"The island has never taken a direct hit from a Category 4 or a Category 5 hurricane in recorded history," he added.
Melissa is the third most intense hurricane observed in the Caribbean after Wilma in 2005 and Gilbert in 1988, according to AccuWeather.
Tánaiste Simon Harris said he strongly advises all Irish citizens in the region to "seek appropriate shelter and to closely follow the advice of Jamaican authorities".
"The Embassy of Ireland to Canada, is accredited to Jamaica. My officials in Dublin and Ottawa, as well as Ireland's Honorary Consul in Kingston are monitoring this evolving situation closely," Mr Harris said.
"We stand ready to provide consular assistance to any Irish people impacted."
Irish citizens in Jamaica who require consular assistance are being asked to contact: +1 613 233 6281.
'People are scared'
Gilbert was the last major storm to directly hit the island.
Colin Bogle, a local adviser to aid group Mercy Corps in Portmore, near Jamaica's capital, said he had heard a loud explosion in the morning, and then everything went dark.
Sheltering with his grandmother, he reported hearing relentless noise and saw trees violently tossed in the wind.
"People are scared. Memories of Hurricane Gilbert run deep, and there is frustration that Jamaica continues to face the worst consequences of a climate crisis we did not cause," he said.
"Food aid will be needed, but recovery support like seeds, tools, and repairs for vehicles will be just as critical to help people restore their livelihoods."
Shortly before landfall, Jamaican electric utility JPS said power outages had affected more than a third of its customers.
In its worst-hit parishes, some three-quarters of customers lost power, JPS said.
Local Government Minister Desmond McKenzie told reporters that nearly 6,000 people had moved into temporary shelters.
The government had issued mandatory evacuation orders for some 28,000 people, but some were reluctant to leave their homes.
"We're getting videos and pictures of severely damaged public infrastructure - hospitals, places of safety ... so it is having the effect that was projected," Jamaican Environment Minister Matthew Samuda told CNN.
"We are happy that there is some weakening but 165 miles per hour - that is still a catastrophe," he added, noting that 70% of the population lived within 5km of the sea.
In Portland Cottage, some 150km south-east of where Melissa made landfall, 64-year-old retiree Collin Henry McDonald told Reuters his community was seeing strong rain and winds, but his concrete roof was holding steady.
"It's like a roaring lion. It's mad. Really mad," he said.
Health authorities in south-eastern Jamaica warned residents to watch out for crocodiles that could be displaced from swamps and rivers and come into residential areas in search of dry land.
Watch: 'Hurricane hunters' fly through eye of Melissa
Jamaica's 'storm of the century'
"It's a catastrophic situation," the World Meteorological Organization's tropical cyclone specialist Anne-Claire Fontan told a press briefing warning of storm surges up to four meters high.
"For Jamaica, it will be the storm of the century for sure."
Nearby Haiti and the Dominican Republic have faced days of torrential downpours leading to at least four deaths, authorities said.
At least three people died during storm preparations in Jamaica, local media reported.
After crossing eastern Cuba, still as a powerful storm, Melissa is forecast to head through the Bahamas, where Prime Minister Philip Davis has ordered evacuations for people in southern and eastern parts of the archipelago.
In Cuba, authorities said they had evacuated upwards of 500,000 people from areas vulnerable to winds and flooding.
"There are no half measures," Cuban President Miguel Díaz-Canel said in a message published in state newspaper Granma, in which he urged people in vulnerable areas to evacuate.
"Melissa will arrive with force, and there's great concern about what it could destroy in its wake," he said.
Storm intensified on approach
Melissa's slow movement over unusually tepid Caribbean water had contributed to its ballooning size and strength, NHC forecasters said, threatening Jamaica with days of never-before-seen catastrophic winds and rain.
Melissa could bring up to 762mm of rain to parts of Jamaica, and up to 305mm to parts of the island of Hispaniola, the NHC said.
The International Federation of the Red Cross (IFRC) said up to 1.5 million people in Jamaica were expected to be directly affected by the storm.
"Today will be very difficult for tens of thousands, if not millions of people in Jamaica," IFRC official Necephor Mghendi said via video link from Port of Spain in Trinidad and Tobago.
To enable swift relief distribution, essential items - tarpaulins, hygiene kits, blankets, and safe drinking water - had been pre-positioned in Red Cross branches on the island, he said, with over 800 shelters set up for evacuees.
'A dire situation unfolding in slow motion'
Melissa made landfall in southwestern Jamaica, which was one of the areas worst-hit by last year's Hurricane Beryl.
Prime Minister Andrew Holness yesterday ordered mandatory evacuations for parts of southern Jamaica, including the historic town of Port Royal.
He warned of damage to farmlands, homes, and infrastructure on the island, which is roughly the size of Connecticut and whose main airports sit close to sea level.
"There is no infrastructure in the region that can withstand a Category 5," he said.
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Mr Holness called on foreign support, saying the government had an emergency budget of $33 million and insurance and credit provisions for damage a little larger than that sustained from Beryl.
Beryl was the earliest and fastest Atlantic hurricane on record to reach Category 5, but scientists warn that storms are becoming stronger and faster as a result of climate change warming ocean waters.
"Slow-moving major hurricanes often go down in history as some of the deadliest and most destructive storms on record," said AccuWeather Chief Meteorologist Jonathan Porter.
"This is a dire situation unfolding in slow motion," he added.
