Hurricane Gonzalo continued to gain strength east of the Caribbean basin Monday, and forecasters said it could become a "major hurricane" in the coming days.

The Miami-based National Hurricane Center said the storm, a Category One on the Saffir-Simpson scale, was 105 miles (170 kilometers) east of St. Thomas at 0300 GMT, packing winds of 85 miles (140 kilometers) per hour.

A hurricane warning was issued for St Martin; Anguilla; and the British Virgin Islands, the center said. That means hurricane conditions are expected in 12 hours' time.

"Additional strengthening is forecast during the next 48 hours and Gonzalo could become a major hurricane by Wednesday," the NHC warned.

A watch was declared for the US Virgin Islands and Puerto Rico, the NHC added. That means Gonzalo could hit there, but is not necessarily expected to do so.