MIAMI, United States (AFP) — Tropical Storm Ernesto advanced Tuesday through the Caribbean toward Puerto Rico, where officials shut down schools and sent workers home as forecasters warned of a possible hurricane.

The storm was moving west toward the US island with maximum sustained winds of 50 miles per hour (85 kilometres per hour), according to the National Hurricane Center (NHC).

It was expected to cross parts of the Lesser Antilles before approaching the US Virgin Islands, British Virgin Islands, and Puerto Rico Tuesday evening.

The storm could become a hurricane within 24 hours, the NHC said — likely after it moves north of Puerto Rico, but with “some risk of the storm becoming a hurricane before that time”.

As a result, a hurricane watch had been issued for the Virgin Islands as well as Culebra and Vieques, two outlying islands off Puerto Rico.

Officials in Puerto Rico have already mobilised the National Guard, suspended classes in public schools, and sent home non-essential workers.

After moving north of the US territory, “additional strengthening is likely”, the NHC said.

Scientists say climate change likely plays a role in the rapid intensification of storms because there is more energy in a warmer ocean for them to feed on.

Warmer than typical Atlantic waters threaten to supercharge this year’s hurricane season, scientists have warned.