Hurricane Gonzalo gained greater strength as it moved over the open Atlantic Tuesday, leaving several people missing in the Caribbean, forecasters said.

Packing maximum sustained winds of 110 miles (175 kilometers) an hour, with gusts even stronger, Gonzalo was right at the top edge of Category Two on the Saffir-Simpson scale and was expected to become a "major hurricane" later in the day, the Miami-based National Hurricane Center said.

The storm skirted the Caribbean and was moving over open ocean, some 390 miles (625 kilometers) east of Grand Turk Island.

But three people were reported missing in the islands of St. Martin and St. Barthelemy after the storm passed, and French authorities expressed concern about four other people they were trying to contact.

The missing were a man who fell off his boat in St. Martin and two others who were trying to get back to their boat in St. Barthelemy.

St Martin is divided between French and Dutch halves, while St. Barthelemy is a French possession.

As the storm moved further from land, tropical storm warnings were discontinued throughout the region, the NHC said.

"Rains associated with Gonzalo are expected to diminish rapidly today," the forecasters added, though potentially "life-threatening" waves and currents would continue in the area and spread to parts of the Bahamas.