Chris was 45 miles (75 km) north-northeast of Barbuda by 8 p.m. EDT (midnight GMT), and moving toward the west-northwest at 13 mph (20 kph) -- a pace that would take it near or over the northernmost Leeward Islands overnight.

The Miami-based hurricane center warned that Chris could grow into a minimum-force hurricane in 72 hours as it neared the Florida Straits on a path that could take it into the Gulf of Mexico, where it could damage key US oil and gas facilities.

The waters of the Gulf are particularly warm, as they were last year when they helped hurricanes Katrina and Rita grow into monster storms before they slammed into the Louisiana and Texas coastlines. Hurricanes need warm water for fuel.

The hurricane center said in a special discussion bulletin posted on its Web site that Chris's winds could reach 65 knots by Friday, just over the 64-knot, or 74 mph (119 kph) threshold at which tropical storms are classified as hurricanes.

The storm's most likely path could take it north of the island of Hispaniola, shared by Haiti and the Dominican Republic, and just to the north of Cuba into the Florida Straits, the hurricane center said.

A tropical storm warning, telling residents to expect storm conditions within 24 hours, was issued for Antigua and Barbuda, St. Kitts and Nevis, Anguilla, Saba, St. Eustatius, St. Barthelemy, St. Martin, Puerto Rico and the US and British Virgin Islands.

The second storm of the season, Tropical Storm Beryl, skirted the North Carolina coast and blew over Nantucket Island and other northeastern tourist playgrounds in July.

The first storm of what is expected to be a busy June to November hurricane season was Alberto, which moved ashore harmlessly in the Florida Panhandle in mid-June.

Forecasters have predicted up to 17 tropical storms and hurricanes this year. Last year saw a record 28, including Katrina, the costliest natural disaster in US history. It devastated New Orleans and other parts of the Gulf Coast and killed more than 1,300 people.