Hurricane Maria has strengthened to the highest level of category five, US forecasters say, as it heads towards the Leeward Islands in the Caribbean.
Maria is moving roughly along the same path as Hurricane Irma, which devastated the region this month.
Hurricane warnings have been issued for Guadeloupe, Dominica, St Kitts and Nevis, Montserrat, Martinique, St Lucia and the US and British Virgin Islands, and a hurricane watch is in effect for Puerto Rico, St Martin, St Barts, Saba, St Eustatius and Anguilla.
Some of these islands are still recovering after being hit by Irma - a category five hurricane which left at least 37 people dead and caused billions of dollars' worth of damage.
US Air Force Reserve Hurricane Hunter aircraft, which fly into storms and hurricanes to gather data, reported Maria had intensified into an extremely dangerous category five hurricane. About 25km from Dominica, it was carrying maximum winds of 260km/h.
In the French territory of Guadeloupe, schools, businesses and government buildings have been closed and severe flooding is predicted in low-lying parts of the islands.
Preparations have also begun in Puerto Rico, where Maria is expected to bring strong winds on Tuesday.
The most southerly point of the Leeward Islands - where Maria will first strike - includes Antigua and Barbuda. The latter island was evacuated after being devastated by Irma.
The NHC said that "a dangerous storm surge accompanied by large and destructive waves will raise water levels by as much as 1.5-2.1m above normal tide levels near where the centre of Maria moves across the Leeward Islands".
It also forecast a maximum potential rainfall of 51cm in some areas of the central and southern Leeward Islands - including Puerto Rico and the US and British Virgin Islands - through to Wednesday night.
"Rainfall on all of these islands could cause life-threatening flash floods and mudslides," it warned.
New Zealander Gareth Butler, who lives on the Turks and Caicos Islands, said the storm was forecast to be category four when it passed within 15-25km of the islands later in the week.
He said there hadn't been time to fully recover from Irma, which had brought "frightening" winds and made a "bit of a mess of the place".
The Puerto Rican government has issued a statement saying it expects the hurricane to make landfall there as a category three on Tuesday.
Virgin Atlantic airline boss Richard Branson, who has a home in the Virgin Islands, has been tweeting ahead of the hurricane's predicted arrival, warning people to stay safe.
In some areas of the British Virgin Islands, entire neighbourhoods were flattened by Hurricane Irma.
Earlier this month, Irma left more than two thirds of homes on the Dutch side of the island of St Martin (known as Sint Maarten) uninhabitable, with no electricity, gas or drinking water.
The French government has said its side of St Martin - known as Saint-Martin - sustained about €1.2 billion in damage, with nine deaths across Saint-Martin and nearby St Barts.
Photo: AFP / NOAA / RAMMB Hurricane Maria was carrying maximum sustained winds of 195km/h as it approached the Caribbean.