Cubans and Americans rushed to flee the increasingly treacherous Hurricane Gustav Saturday, as officials warned the storm is poised to worsen into the strongest possible category of hurricane as it crosses the Gulf of Mexico.

Jeffrey Vannor carries his belongings at the Greyhound and Amtrak station in New Orleans while fleeing the approaching Hurricane Gustav Saturday. (Rob Carr/Associated Press)

United States Federal Emergency Management Agency chief David Paulison said during a media briefing Saturday afternoon that the storm was strengthening into a Category 5 hurricane on the Saffir-Simpson Hurricane Scale.

Hurricanes that reach Category 5 are in the highest classification of tropical cyclone and are considered to be the most catastrophic, with winds greater than 250 km/h.

Officials said the storm, which lashed parts of Cuba Saturday before beginning its journey toward the oil-rich Gulf of Mexico by evening, could reach Category 5 strength by early Sunday.

The storm is expected to make landfall Monday along the U.S. Gulf Coast, where a hurricane watch has been issued from Texas east to the Florida-Alabama border.

In U.S., 1 million flee coastal cities

Anticipating Gustav's ferocious arrival, more than one million Americans fled New Orleans and other coastal cities Saturday.

In New Orleans, where Hurricane Katrina killed about 1,600 people three years ago, Mayor Ray Nagin said a mandatory evacuation of the city's West Bank district will come into effect 8 a.m. local time Sunday. The same order becomes mandatory for the East Bank district at noon.

Traffic backs up along westbound Interstate 10 as residents of the New Orleans area leave the region in anticipation of Hurricane Gustav. (Brian Lawdermilk/Associated Press)

Describing Gustav as the storm of the century, Nagin told people during a news conference to "get your butts out of New Orleans now," noting that the city will not provide emergency services to those who stay behind.

"This is the real deal, not a test," Nagin said as he issued the evacuation order, warning residents that staying would be "one of the biggest mistakes of your life."

Officials began putting an estimated 30,000 of the city's most vulnerable residents, including elderly, disabled and low-income individuals, on buses and trains Saturday morning. Nagin also recommended all tourists leave town.

U.S. President George W. Bush made telephone calls Saturday to governors and federal officials Saturday to see if Washington could provide any help, designating two additional states — Mississippi and Alabama — eligible for federal help before Gustav makes landfall.

"That doesn't mean that everything will be totally smooth," White House press secretary Dana Perino said. "We're facing what could be a very strong hurricane, possibly one of the largest and strongest to hit America since records began."

Presumptive Republican presidential nominee John McCain said he may cancel next week's party convention if the U.S. is badly affected by the hurricane.

300,000 forced to evacuate in Cuba

By late Saturday, Hurricane Gustav was already a dangerous Category 4 storm as it lashed Cuba's tobacco-growing western tip. Just hours earlier, Gustav battered Cuba's Isla de Juventud, or Isle of Youth, with 240 km/h winds that toppled trees and telephone poles.

"The rain is not so intense, but there is a lot, a lot of wind," said Isabel Alarcon, a resident of Nueva Gerona, the largest city on the island of 87,000 people.

"The officials, they have told us the wind will be bad first, but then the rain could cause flooding into the night," she said Saturday.

A couple carrying a child in Pinar del Rio, Cuba, walk along a street during rains caused by the approaching of Hurricane Gustav Saturday. (Javier Galeano/Associated Press)

The island's civil defense chief, Ana Isla, said many people had been injured, and nearly all the roads on the island had been washed out.

About 300,000 residents were forced to evacuate their homes in western Cuba and Isla de la Juventud.

The storm rolled toward Cuba from the Caribbean, where it has killed 81 people. Haiti's Interior Ministry on Saturday raised the hurricane death toll there to 66 from 59, while Jamaica upped its death toll to seven from four. Gustav also killed eight people in the Dominican Republic.

Bill Read, the head of the U.S. National Hurricane Center, interrupted an afternoon teleconference involving disaster and weather officials on Saturday to announce the Centre was going to issue a special advisory statement raising Gustav to Category 5, according to FEMA officials.

"That puts a different light on our evacuations and hopefully that will send a very clear message to the people in the Gulf Coast to really pay attention," Paulison said.

More than three-quarters of the Gulf of Mexico's oil production and 40 per cent of its natural gas output was shut down by Saturday afternoon as oil companies evacuated their facilities, according to the U.S. Minerals Management Service.

Analysts warned that ongoing supply disruptions could prompt a price spike for gasoline and other petroleum products.

Meanwhile, Tropical Storm Hanna is expected to approach Turks and Caicos Islands late Sunday or Monday, before it winds through the Bahamas — and possibly onto Cuba — sometime next week.

With files from the Associated Press