The U.S. National Hurricane Centre's forecast indicated Irene, now classified as a Category 2 storm, may become a major Category 3 storm, with winds over 111 mph (178 kph), before hitting the southeast U.S. coast by the weekend.
Irene, the ninth named storm of the busy 2011 Atlantic season, looks set to be the first hurricane to hit the United States since Ike savaged the Texas coast in 2008.
Hurricane-force winds extended outward from the core to 50 miles (85 km) and tropical storm-force winds extended out up to 205 miles (335 km).
Earlier, Irene buffeted Puerto Rico with winds and heavy rain, knocking out power and downing trees.
While the core of the storm was expected to stay out to sea as it moved past Florida, Irene was wide enough for its outer squalls to reach the Florida shore.
A strengthening Hurricane Irene is lashing the northern coast of the Dominican Republic and it was moving towards the west-northwest on Tuesday morning.
At 5 a.m., The National Hurricane Center reported that at a Category 2 storm, Irene sustained winds near 100 mph. Forecasters issued a hurricane watch for the northern coast of Haiti from Le Mole St. Nicholas eastward to the border. Turks and Caicos Islands, the Bahamas and the rest of Haiti are under hurricane warning.
The impact could be serious wherever Irene makes landfall. With little but warm water and favorable winds in its path, forecasters expected the storm to steadily intensify. Along with gusts and heavy rains, Irene also will be pushing a five- to eight-foot storm surge into the southeastern Bahamas that could reach seven to 11 feet by the time it reaches the Central Bahamas.
Even as a tropical storm, Irene proved damaging. The island of Vieques remained completely without power.
Irene grew into a Category 2 hurricane late yesterday and the U.S. National Hurricane Center in Miami said it could reach Category 3 today and possibly become a monster Category 4 storm within 72 hours.
National Hurricane Centre spokesman Dennis Feltgen urged south Florida residents to monitor the storm carefully, to double check their supplies of food and fuel and to review their hurricane plans.
Officials in Charleston, South Carolina, also warned residents to monitor Irene closely. It has been six years since a hurricane hit the South Carolina coast, said Joe Farmer of the state Emergency Management Division.
Earlier, the storm slashed directly across Puerto Rico, tearing up trees and knocking out power to more than a million people, then headed out to sea north of the Dominican Republic, where the powerful hurricane's outer bands were buffeting the north coast with dangerous sea surge and downpours.
Irene is the first hurricane of the Atlantic storm season.
The heavily-populated south east Florida coast is in the direct line of numerous forecast cones.
However, the NHC stressed Irene’s path still remains uncertain and it could make landfall anywhere from the Florida Keys to the Carolinas.
Warm sea water is also expected to fuel the storm.
Puerto Rico took a pounding yesterday morning with at least five rivers bursting their banks.
The fast-changing storm has downed trees and caused widespread power cuts in the U.S. Virgin Islands as it shifted north over Caribbean territory.
It is feared the hurricane could cause dangerous mudslides and floods in Dominican Republic, the NHC said.
Residents of Antigua reported rains, strong squalls and surf as the storm passed the Leeward Islands in the north-east corner of the Caribbean.