Irma strengthened Tuesday into an "extremely dangerous" Category Five hurricane, meteorologists warned, sparking alarm and flooding alerts as it barrelled towards the Caribbean.
The monster hurricane coming on the heels of Harvey, which struck Texas and Louisiana late last month is expected to make landfall along the string of French islands including Guadeloupe late Tuesday before heading to Haiti and Florida.
The Miami-based National Hurricane Center in its 1200 GMT bulletin said Irma had strengthened to the most powerful Category Five, packing winds of 175 miles (280 kilometers) per hour.
The front was moving west at 14 miles (22 kilometers) per hour, and is expected to drop between four and eight inches (10 and 20 centimeters) of rain when it hits land.
"These rainfall amounts may cause life-threatening flash floods and mudslides," the NHC warned.
Irma's centre was located about 320 miles (515 km) east of the West Indies' Leeward Islands, the NHC said, urging that "preparations to protect life and property should be rushed to completion" in the region.
Schools and government offices in Guadeloupe have been ordered shut, while hospitals are stocking up on medicines, food and drinking water. People living on shorelines will be moved to safety, authorities said.
Saint Barthelemy and St Martin islands, both popular holiday destinations, are expected to be especially hard hit.
The top French official of the islands, Anne Laubies, said in Saint Martin's capital Marigot the hurricane posed the greatest threat in 20 years, with more people endangered in flood-prone areas because of a rise in population.
The governor of the US state of Florida, Rick Scott, declared a state of emergency, saying Irma posed "a severe threat to the entire state of Florida", barely a week after Harvey claimed at least 42 lives.
Long queues of people rushed to get batteries and bottled water, while many cut trees around their dwellings and sought to tie down objects and seal up their windows.
Category Five hurricanes are rare and are capable of inflicting life-threatening winds, storm surges and rainfall.
A hurricane of this magnitude can tear off roofing, shatter windows, uproot palm trees and turn them into projectiles that can kill people.
Irma is projected to make landfall late Tuesday or early Wednesday, bringing water levels up to nine feet (3 metres) above normal levels, rainfall of up to 10 inches (25 centimetres) in areas, and "large and destructive waves," the US National Hurricane Center warned.
Harvey, which dumped as many as 50 inches of rain in some parts of Houston, turning neighbourhoods into lakes and causing material damage estimated at around $100 billion (85 billion euros), was a Category Four hurricane.
In Puerto Rico, a US territory of 3.5 million, Governor Ricardo Rossello activated the National Guard and announced the opening of storm shelters able to house up to 62,000 people.
The mayor of the Puerto Rican capital San Juan, Carmen Yulin Cruz Soto, ordered 900 municipal employees -- police, emergency personnel, and aid and social workers -- to report for rotating 12-hour shifts.
Even if Puerto Rico is spared a direct hit, the mayor said, three days of pounding rain will do heavy damage.
A US aircraft carrier with a field hospital and dozens of aircraft able to conduct rescue or supply missions has been positioned protectively in the area, according to Alejandro de la Campa of the Caribbean division of the US Federal Emergency Management Agency (FEMA). Local press identified the carrier as the USS Kearsarge.
Irma's precise path remains unclear. But several projection have it passing over the Dominican Republic, Haiti and Cuba before turning north toward Florida and then possibly swinging up the US East Coast.
Irma is expected to produce total rainfall accumulations of 3 to 6 inches across the islands of the northeastern Caribbean, with isolated maximum amounts of 10 inches across the northern section.