Hurricane Irma strengthened into a highly dangerous Category 5 storm as it barrelled towards the Caribbean and the southern United States.
Hurricane warnings were in effect for much of the Leeward Islands, the British and US Virgin Islands and Puerto Rico as the storm packed winds of up to 280km/h, the US National Hurricane Centre (NHC) said.
Category 5 is the highest NHC designation.
The centre urged that "preparations should be rushed to completion in the hurricane warning area."
Irma was about 440km east of the island of Antigua and moving west at about 22km/h, the NHC said.
It was too early to forecast Irma's exact path or any effects have on the continental United States, it cautioned.
Puerto Rican Governor Ricardo Rossello urged the 3.4 million residents of the US territory to leave areas expected to be lashed by high winds and torrential rains as early as tonight and seek refuge in one of 460 hurricane shelters.
"This is something without precedent," Mr Rossello told a news conference.
He will ask US President Donald Trump to declare a federal state of emergency even before the storm passes to allow disbursement of US emergency funds.
Florida Governor Rick Scott said he also asked Mr Trump to make a "pre-landfall" emergency declaration.
Irma is expected to reach southern Florida on Saturday.
Mr Scott said he spoke with Mr Trump last night after the governor declared a state of emergency for all of Florida's 67 counties.
He said the president "offered the full resources of the federal government."
Irma would be the second powerful hurricane to thrash the US in as many weeks.
Residents of Texas and Louisiana were still reeling from Hurricane Harvey, which struck Texas as a Category 4 hurricane on 25 August.
It dumped several feet of rain, destroying thousands of homes and businesses, killing an estimated 60 people and displacing more than 1 million others.
