People look at a swollen river in Roseau, the capital of the Caribbean island of Dominica, after it was hit by Hurricane Dean. (AFP)

Kingston, Aug. 18 (Reuters): Hurricane Dean was on the verge of becoming a rare Category 5 storm today as it roared toward Jamaica and the energy-rich Gulf of Mexico after hammering the eastern Caribbean, where it was blamed for at least three deaths.

Millions of people were under storm alerts in some of the most populous areas of the Caribbean, including parts of vulnerable Haiti and its teeming capital, Port-au-Prince, the Dominican Republic, Cuba and mountainous Jamaica which was in the direct path of the powerful hurricane.

With sustained winds of 240 kmph and gusts over 300 kmph, Dean was a Category 4 storm, the second highest level on the five-step Saffir-Simpson scale of hurricane intensity.

It was expected to become a Category 5 storm within two days, with sustained winds of more than 250 kmph.

Jamaica’s political parties suspended campaigning for August 27 national elections as residents prepared for Dean. Lines formed at petrol stations and people crowded markets, emptying shelves of batteries, canned tuna, rice and bottled water. “The country is on high alert,” said Kerry-Ann Morris, a spokeswoman for Jamaica's disaster preparedness office. “Hurricane Ivan three years ago is a very strong memory for a lot of Jamaicans and it was a very scary moment.”

Dean’s progress was being watched by energy markets, which have been roiled by hurricanes since powerful storms in 2004 and 2005 disrupted oil and gas production.

Energy firms evacuated workers from offshore rigs in the Gulf of Mexico, home to a third of US domestic crude production.

The latest computer models showed Dean hitting Mexico’s Yucatan Peninsula around Tuesday before emerging in the Gulf of Mexico. Most had the storm hitting the Mexico coast but one took it ashore in southern Texas on Wednesday or Thursday.

Dean’s destructive core was expected to stay off Haiti’s south coast. But tropical cyclones frequently trigger deadly flash floods and mudslides in the deforested, poverty-stricken country of 8 million people. A brush with tropical storm Jeanne in 2004 killed nearly 3,000.