Hurricane Irma strengthened into a highly dangerous Category 5 storm on Tuesday as it barreled toward the Caribbean and the southern United States, threatening deadly winds, storm surges and flooding as Texas and Louisiana was still reeling from devastating Hurricane Harvey.

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Hurricane warnings were in effect for much of the Leeward Islands, the British and U.S. Virgin Islands and Puerto Rico ahead of the storm packed maximum sustained winds of nearly 185 miles per hour, with higher gusts, on Tuesday, the U.S. National Hurricane Center (NHC) said.

Monroe County, Florida, home to the Florida Keys, on Tuesday issued mandatory evacuation orders for tourists and residents alike, warning that no safe shelter would be available on the islands.

"All visitors, tourists and non-residents are hereby urged to seek safe shelter in mainland Florida," according to the Monroe County's warning.

Visitors must start evacuating by 7 a.m., ET, on Wed., Sept. 6; residents were told to start leaving by 7 p.m. on the same day.

"If ever there was a storm to take seriously in the Keys, this is it," Monroe County Emergency Management Director Martin Senterfitt said, in the note. "The sooner people leave, the better."

Irma marked a serious milestone Tuesday, becoming the strongest hurricane in the Atlantic basin outside of the Caribbean Sea & Gulf of Mexico in NHC records.

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"Irma becomes an extremely dangerous Category 5 hurricane," the NHC said on Tuesday, adding that it could gain even more strength. "Preparations should be rushed to completion in the hurricane warning area."

Puerto Rican Gov. Ricardo Rossello, who has declared a state of emergency and activated the National Guard, urged the economically struggling U.S. territory's 3.4 million residents to prepare for the potentially devastating storm.

"There is no positive sign that it's going to go in another direction. We're expecting that it's coming at Puerto Rico with force, and we've got to be ready for it," he said at a news conference.

Irma was about 130 miles east of the island of Antigua and moving west at about 15 mph, the NHC said.

Lisa Ferguson, the owner of the Esperanza Inn on the Puerto Rican island of Vieques, said her 15 guests left on Sunday and that staff put up hurricane shutters for the storm's expected passage to the north on Wednesday.

But there no signs of a looming hurricane on Tuesday, with clear skies, a fresh breeze and singing birds, she said.

"It's kind of ominous, knowing that it's out there. But it's perfect," Ferguson said by telephone. "We're bunkering down."