Irma headed towards Puerto Rico, Haiti, Cuba and Florida, with potential for coastal storm surges of up to six metres.
Hurricane Irma has ploughed past the Dominican Republic towards Haiti after devastating a chain of Caribbean islands and killing at least 10 people.
With winds of around 290 kilometres per hour, the Category Five superstorm is one of the most powerful Atlantic storms in a century.
On Thursday, it lashed several small islands in the northeast Caribbean, including Barbuda, Saint Martin and the British Virgin Islands, tearing down trees, flattening homes and causing widespread damage.
The eye of the hurricane did not directly hit Puerto Rico, passing north early on Thursday, battering the US territory with high winds and heavy rains.
Three people were killed, and around two-thirds of the population lost their electricity, Governor Ricardo Rossello said.
Far out in the Atlantic, Hurricane Jose grew into a Category 2 storm, threatening some of the same islands ravaged by Irma. In the Gulf of Mexico, Hurricane Katia was virtually stationary on Thursday afternoon.
Al Jazeera’s Andy Gallacher, reporting from Puerto Rico’s capital, San Juan, said: “As things stand, there are three hurricanes in the Atlantic – Irma, Jose and Katia – which hasn’t happened since 2010. But, it’s Irma that poses the biggest threat, with the UN saying 37 million could be affected.”
He added: “In the coming hours and days Irma’s course will be closely monitored but its track seems set and the impact could be catastrophic.”
The small island of Barbuda is said to be “barely habitable” after being hit by the storm, leaving about 60 percent of its 1,400-strong population homeless.
“Either they [buildings on the island] were totally demolished, or they would have lost their roof,” said Gaston Browne, Antigua and Barbuda prime minister.
Irma has already “battered” 1.2 million people, the Red Cross said on Thursday, warning the storm could upend the lives of as many as 26 million in the coming days.
The eye of the storm was moving west-northwest off the northern coast of the Dominican Republic, heading slightly north of Haiti, the National Hurricane Center (NHC) said.
The United Nations Children’s Fund (UNICEF) warned that millions of children could be at risk in the two countries, which share the island of Hispaniola.
Impoverished Haiti has been particularly vulnerable to hurricanes and heavy rains.
Nearly 900 people were killed by Hurricane Matthew last year.
Haiti’s government has put the country on alert, closing schools and moving people to shelters.
“There is a big preparation effort going ahead in Haiti, with more than 700 shelters in place ahead of the arrival of Irma,” said Al Jazeera’s John Holman, reporting from the capital, Port-au-Prince.
Ascension Martinez, Save the Children’s director of programme quality and advocacy in Haiti, said that health centres and schools had become emergency contact points.
“This hurricane will mean heavy rains and floods for Haiti, where the poorest communities are still suffering from the consequences of the last hurricane to hit the country, just under a year ago,” said Martinez.
Irma will likely hit the US state of Florida as a very powerful Category Four storm on Sunday, with storm surges and flooding beginning within the next 48 hours, according to the NHC.
Mass evacuations are under way in Florida as forecasters predict Irma has the potential to slam into the southern US state with high winds and huge tidal surges.
“Every Floridian, every family should take this seriously and be aggressive,” said Rick Scott, governor of Florida. “This is serious, and we cannot take chances.”
He added: “It is life threatening. This is not a storm you can sit and wait through.”
Meanwhile, Cuba started evacuating some of the 51,000 tourists visiting the island, particularly 36,000 people at resorts on the picturesque northern coast, most of them Canadians.
The Bahamas also lie directly in the path of the storm. Its government says it will shut the main international airport in Nassau before the arrival of Irma.