Tropical Storm Irma in the eastern Atlantic Ocean. REUTERS/NASA/NOAA

(Reuters) - Hurricane Irma on Thursday strengthened into a Category 3 hurricane over the eastern Atlantic and was headed towards the Caribbean, the U.S. National Hurricane Center (NHC) said in its latest advisory.

The hurricane was about 720 miles (1,160 km) west of the Cabo Verde islands, packing maximum sustained winds of 115 miles per hour (185 kph), the Miami-based weather forecaster said.NHC forecast models were showing it heading for the U.S. territory of Puerto Rico, the Dominican Republic and neighboring Haiti with possible landfall by the middle of next week.