Washington, Sep 1 (IANS): A powerful hurricane named Irma is rapidly intensifying in the open Atlantic and is posing a major threat to the Caribbean and the US, weather officials said.

Irma was named as a tropical storm on Wednesday morning and by Thursday afternoon it had strengthened into a large Category 3 hurricane, with winds of 115 mph, CNN quoted the National Hurricane Centre (NHC) as saying.

It may hit the Caribbean on September 6.

Such explosive strengthening is known as "rapid intensification", defined by the NHC as having its wind speed increase at least 30 knots in 24 hours.

"Irma has become an impressive hurricane," the NHC said on Thursday, noting the rapid intensification.

The catastrophic hurricane Harvey underwent rapid intensification last week, just before it made landfall in Texas on August 25 which brought it from a tropical storm to a Category 4 hurricane.

Harvey has flooded major parts of Texas including the city of Houston. Over 40 people have been killed and millions affected by it.

Irma is forecast to continue to strengthen as it moves westward over the next five days. It will turn into a dangerous Category 4 hurricane by the time it hits the Caribbean next week.