As many as 37 million people could be affected by Hurricane Irma, the United Nations has warned.
The Category 5 hurricane is ripping its way through the Caribbean and heading towards the southern US state of Florida.
It has already killed at least six people, including a two-year-old.
UN spokesman Stephane Dujarric said the UN has sent a humanitarian team to the island of Barbados to work with the Caribbean Disaster Emergency Management Agency to help hurricane victims.
Additional teams are on standby, he added.
The islands of Antigua and Barbuda were the first to be hit by Irma.
There has been no communication from Barbuda since the hurricane arrived but Prime Minister Gaston Browne said 95% of buildings had been flattened. Around 60% of the roughly 1,400 residents have been left homeless.
St Martin and Anguilla experienced heavy rain and winds of up to 185mph, while damage in St Barts was described as “apocalyptic” after winds of 151mph.
Irma is now moving over Puerto Rico where 100mph winds have left at least 600,000 people without power and nearly 50,000 without water.
Puerto Rico’s economic crisis has resulted in major funding and staffing cuts to the public power company and, consequently, the power supply may not be restored for up to six months.
The most powerful Atlantic storm ever recorded is then due to hit the Dominican Republic, Haiti, Cuba and the Bahamas, before being expected in Florida by Sunday.
In Florida, people in low-lying areas of highly populated Miami-Dade County were urged to move to higher ground in preparation for the storm.
Roads were packed with vehicles and there were petrol shortages, prompting Governor Rick Scott to tell people to only “take what they need”.
Mr Scott has described the storm as “life-threatening”, telling people to follow evacuation orders because “you can rebuild your home – you cannot rebuild your life”.
There are fears that dozens of cranes being used on construction sites may topple over, as they are built to withstand winds of up to 145mph but not the force of winds expected from Category 5 Irma.
Irma will likely remain at Category 4 or 5 for at least the next day or two, according to the National Hurricane Center.
Meanwhile, two other hurricanes are in the same region – the first time this has happened since September 2010.
Katia formed in the Gulf of Mexico with winds of 75mph and is expected to drift towards the coast of Mexico on Thursday, prompting the government to issue a hurricane watch for the coast of Veracruz state.
Hurricane Jose is further east than Irma and, while meteorologists said it was not an immediate threat, it is strengthening and its path could change.