MIAMI: Tropical Storm Fred, the sixth of this year's Atlantic hurricane season, formed south of Puerto Rico and was expected to move westwards through the Caribbean, the US National Hurricane Center (NHC) said early Wednesday.

The cyclone was about 40 miles (65 kilometers) southwest of Puerto Rico, the NHC said.

Tropical storm warnings have been issued for Puerto Rico, the US Virgin Islands and the Dominican Republic, where the cyclone is expected to hit in the next 12 hours.

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Haiti, the Turks and Caicos Islands and the southeast Bahamas were under storm watch.

"On the forecast track, the center of Fred is expected to be near or over Hispaniola (the island shared by Haiti and the Dominican Republic) later today, and move near the Turks and Caicos Islands and the southeastern Bahamas on Thursday," the NHC said.

Fred was moving west at around 17 miles per hour with maximum sustained winds of around 40 mph with higher gusts.