CHARLOTTE AMALIE, US Virgin Islands (AP) -  The outer bands of strengthening Tropical Storm Rafael drenched the Leeward Islands on Sunday as its centre moved over open ocean on a track toward Bermuda.

There were no immediate reports of any significant damage or injuries in the islands as the centre of the tropical storm churned toward the north-northwest. By late Sunday morning, it was about 170 miles (275 kilometres) northeast of San Juan, Puerto Rico, and was packing winds of roughly 60 mph (95 kph).

Forecasters at the US National Hurricane Centre in Miami said Rafael could strengthen into a hurricane by late Monday as it spins over the open Atlantic.

Rafael is expected to approach the wealthy British Atlantic territory of Bermuda on Tuesday. The Bermuda Weather Service issued a tropical storm watch for the storm-hardened territory of about 70,000 people late Sunday morning.

In the US Virgin Islands, Elton Lewis, the territory's emergency management director, said authorities "don't anticipate any significant impact" from the passing storm.

Tropical storm warnings have been discontinued for Caribbean islands. But the possibility of flooding and mudslides are still a concern in some islands, particularly in mountainous terrain, since Rafael has been forecasted to dump between four to six inches of rain over the Lesser Antilles, small islands that include Dominica, Martinique and St. Lucia.

Rain ranging from one to three inches was expected in the US Virgin Islands and the outlying Puerto Rican islands of Culebra and Vieques.