(Updates position, wind, adds space shuttle returning early)

By Carole Beckford

KINGSTON, Aug 18 (Reuters) - Residents snapped up emergency supplies and tourists crowded Caribbean airports on Saturday as Hurricane Dean bore down on Jamaica and the Cayman Islands and threatened to pound Mexico's Yucatan Peninsula as a rare Category 5 storm.

The first hurricane of what is expected to be an above-average 2007 Atlantic storm season had already battered the eastern Caribbean, where it killed at least three people.

At least one more person died on Saturday and millions went on alert in some of the most populous areas of the Caribbean, including Haiti, the Dominican Republic, Cuba, all of mountainous Jamaica and the wealthy British territory of the Cayman Islands, a major financial center.

"If we don't manage to leave we'll go back to the hotel and barricade the hotel room and then hope and pray," said Dutch tourist Gideon Tuttezs as he waited in a standby queue to fly out of Jamaica's Montego Bay.

With sustained winds of 145 mph (230 km per hour), Dean was a Category 4 storm, the second-highest on the five-step Saffir-Simpson scale. Its winds had dipped slightly but its minimum central pressure -- an indicator of potential strength -- was dropping ominously and it was expected to smash into Jamaica on Sunday as an exceptionally dangerous storm.

It could become a Category 5 storm after roaring by the Caymans in two days, with winds of over 155 mph (250 kph).

At 11 p.m. EDT (0300 GMT), Dean was located 360 miles (580 km) east-southeast of Kingston and about 170 miles (275 km) south-southeast of Port-au-Prince. It was moving west at 17 mph (28 kph), the U.S. National Hurricane Center said.

Jamaica's government urged people to flee low-lying and landslide-prone areas, buses were marshaled to transport evacuees and police and troops were put on alert.

"Maybe I am hoping for too much, but I only wish that the hurricane will change course," said Kingston resident Matthew Turner.

Lines formed at gas stations and supermarkets were crammed as shoppers bought batteries, flashlights, canned tuna, rice and water. Campaigning for Aug. 27 elections was halted.

SWEPT OUT TO SEA

Officials in the Dominican Republic, where the hurricane sent 18-foot (5.5-metre) waves crashing onto southern beaches, first reported three deaths but later retracted two. They said a 16-year-old Haitian was swept out to sea by a wave.

Dean's progress was being watched closely by energy markets, skittish since powerful storms in 2004 and 2005 swept through the Gulf of Mexico where roughly a third of U.S. domestic crude is produced. At least one production platform and two oil rigs were evacuated.

The latest computer models showed Dean slamming into Mexico's Yucatan Peninsula early in the week and then hitting the northern Mexico coast rather than threatening the most critical U.S. offshore oil and gas production areas.

Mexican authorities began evacuations on the Caribbean coast, while U.S. President George W. Bush issued an emergency declaration for Texas to free up federal help and funds.

NASA prepared to bring the U.S. space shuttle Endeavour, which has been on a mission to the International Space Station, back to Earth a day early on Tuesday just in case Mission Control in Houston had to close because of the storm.

Dean's destructive core was expected to spare Haiti. But tropical cyclones frequently trigger flash floods and mudslides in the deforested, poverty-stricken country of 8 million. A brush with Tropical Storm Jeanne in 2004 killed nearly 3,000.

In the Cayman Islands, which were swamped and battered by Category 4 Hurricane Ivan in September 2004, national airline Cayman Airways laid on extra flights to Miami.

"I'm scared. We're trying to get out," said resident Ben Webster, who was due to leave for holiday with his wife and daughter on Wednesday. "We'd rather go sooner than later."

Dean trampled Martinique, St. Lucia and Dominica on Friday, triggering landslides, lifting roofs off houses, knocking out power and destroying banana and sugar cane plantations. Three people were killed in Dominica and St. Lucia.

Category 5 hurricanes are rare. Until the record-breaking 2005 Atlantic hurricane season, records showed only two years -- 1960 and 1961 -- with more than one Category 5 storm.

But in 2005, four hurricanes reached that strength -- Emily, Katrina, Rita and Wilma -- triggering debate about the impact of global warming on tropical cyclones. (Additional reporting by Jim Loney, Michael Christie and Jane Sutton in Miami, Horace Helps and Carlos Barria in Kingston, Manuel Jimenez in the Santo Domingo, Shurna Robbins and Alan Markoff in George Town and Anna Willard in Paris)

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