Hurricane Irma, barrelling toward the Caribbean and Florida, was upgraded to a powerful Category 4 storm on Monday as islands in its path braced for a possible onslaught.

Hurricane advisories were issued for territories that dot the West Indies, including parts of the Leeward Islands, the British and U.S. Virgin Islands and Puerto Rico, in preparation for the intensifying storm that could pummel the area with life-threatening wind, storm surges and torrential rain by Tuesday evening, according to the U.S. National Hurricane Center.

Puerto Rico Gov. Ricardo Rossello declared a state of emergency and activated the National Guard on Monday in preparation for the landfall of Irma as it was upgraded.

"Despite the economic challenges Puerto Rico is facing, the approved budget has $15 million for the emergency fund," Rossello said in a statement.

Florida Gov. Rick Scott also declared a state of emergency on Monday, pressing the need for his state to get ready for Irma.

"Hurricane Irma is a major and life-threatening storm and Florida must be prepared," Scott said in a statement.

"This state of emergency allows our emergency management officials to act swiftly in the best interest of Floridians without the burden of bureaucracy or red tape."

Irma is set to hit the Puerto Rico on Wednesday, the National Hurricane Center (NHC) said.

The NHC had forecasted Irma would strengthen on Tuesday night, but it said Monday it had developed into a Category 4 hurricane.

To help residents prepare for the storm, the Puerto Rican government activated a price freeze on basic necessities, including food and water, medicines, power generators and batteries.

The U.S. island territory, home to about 3.4 million people, has 456 emergency shelters prepared to house up to 62,100 people.

Telemundo TV station WIPR in Puerto Rico showed long lines of shoppers stocking up on bottled water, flashlights, batteries, generators, food and other items.

The storm will be the second powerful hurricane to thrash the United States and its territories in as many weeks.

Residents in Texas and Louisiana are still reeling from the devastating effects of deadly Hurricane Harvey, which dumped several feet of rain and destroyed tens of thousands of homes and businesses when it landed on Aug. 25.

The intensifying Hurricane Irma threatens to pummel the Caribbean, Florida and the U.S. East Coast, according to the NHC, which cautioned that it was still too early to forecast its exact path or what effects it might have on the continental U.S.

"There is an increasing chance of seeing some impacts from Irma in the Florida Peninsula and the Florida Keys later this week and this weekend. In addition, rough surf and dangerous marine conditions will begin to affect the southeastern U.S. coast by later this week," the centre said.