Miami, Florida - Hurricane Omar swept across the northern Leeward Islands after strengthening to a category 3 storm in the Caribbean with maximum sustained winds of 195 kilometres per hour and higher gusts, the National Hurricane Center in the United States said.

It gained in intensity on the five-step Saffir-Simpson Scale before hitting the Antilles and beginning its march late Wednesday across the Leeward Islands.

Omar was moving north-east at 32 kmph, and its eye was expected to pass between the Virgin Islands and the northern Leeward Islands overnight, the centre in Miami, Florida, said.

Hurricane warnings were issued for the US Virgin Islands, the Puerto Rican islands of Vieques and Culebra, St Martin, Saba, St Eustatius, St Barthelemy, the British Virgin Islands and Anguilla.

Residents there were warned to take measures to protect their lives and property as hurricane conditions were expected within the next 24 hours.

The National Hurricane Center forecast rainfall of 13 to 50 centimetres, warning that such precipitation could produce flash floods and mudslides.

It also expected dangerous waves and coastal storm surges of 1.2 to 1.8 metres.

Omar is the 15th named storm and the seventh hurricane this year in the Atlantic. (dpa)