Gonzalo gained strength as a major Category Four hurricane Wednesday, packing strong winds as it headed over the Atlantic toward Bermuda, US forecasters said.

A hurricane watch went up in Bermuda, which could feel the storm's fury by Friday, the Miami-based US National Hurricane Center said.

It was already causing large swells in the Virgin Islands, Puerto Rico and the Dominican Republic.

Gonzalo has been growing in power since skirting the Caribbean and heading into the open Atlantic Tuesday on a track well off the North American mainland but projected to pass near Bermuda.

The hurricane center declared the storm a Category Four on the five-point Saffir-Simpson scale, meaning its high winds would be "extremely dangerous" if it touches land.

At 1500 GMT, the storm's top winds had reached 130 miles (210 kilometers) per hour. It was located 640 miles southwest of Bermuda and was moving northwest at 12 miles per hour, the NHC said.

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.

The storm caused property damage on both islands, which were battered by strong winds and heavy rains.

In St Barthelemy, a plane flipped on the airport's runway, while roads were blocked by fallen trees and telephone and power lines were torn from the ground.

Gonzalo is the seventh storm of the Atlantic season -- which stretches from June to November -- and the third hurricane to slam the Caribbean this year.

Hurricane Cristobal left at least four people dead in late August when it trashed the Bahamas, Turks and Caicos Islands and Dominican Republic with heavy rains causing serious flooding.

In September, Hurricane Edouard remained too far from land to cause any serious damage.

The NHC predicted that storm activity will be lower than average this year.