Tulum, Mexico ? Hurricane Dean strengthened into a monstrous Category 5 storm Monday night as its outer bands of wind and rain slammed the coasts of Mexico and Belize. Thousands of tourists fled the beaches of the Mayan Riviera as it roared toward the ancient ruins and modern oil installations of the Yucatan Peninsula.

Mexico’s state oil company, Petroleos de Mexico, said it was evacuating all of its more than 18,000 offshore workers in the southern Gulf of Mexico, which includes the giant Cantarell oil field. Dozens of historically significant Mayan sites also were emptied.

Dean, which has killed at least 12 people across the Caribbean, quickly picked up strength after brushing Jamaica and the Cayman Islands.

By 10 p.m. CDT, Dean had sustained winds of 160 mph and was centered about 150 miles east of Chetumal, where it was projected to make landfall early today, according to the U.S. National Hurricane Center said. Chetumal is about 120 miles south of Tulum.

Category 5 storms – capable of catastrophic damage – are extremely rare. Only three have hit the U.S. since record-keeping began.

Cancun seemed likely to be spared a direct hit, but visitors abandoned its swank hotels to swarm outbound flights. Officials evacuated more rustic lodgings farther south.

Eric Morovich of Orange County, Calif., waited outside Cancun’s airport after trying unsuccessfully to book a ferry, rent a boat and charter an airplane. “The next option is swimming, I guess,” he joked.

A hurricane warning was in effect from Cancun all the way south through Belize. All hospitals were closed in Belize City, the country’s biggest, and authorities urged residents to leave, saying Dean is too strong for their shelters. Meteorologists said a storm surge of 12 to 18 feet was possible at the storm’s center.

The storm was expected to slash across the Yucatan and emerge in the Gulf of Campeche, where Petroleos de Mexico decided Monday to shut down production on the offshore rigs that extract most of the nation’s oil.

Shutting the 407 oil wells in the Campeche Sound will result in a production loss of 2.7 million barrels of oil and 2.6 billion cubic feet of natural gas a day, Pemex said. Of that, about 1.7 million barrels of oil a day is exported from three Gulf ports, where Pemex was loading the final tankers before shutting them as well.

At the southern tip of Texas, officials urged residents to evacuate ahead of the storm. “Our mission is very simple. It’s to get people out of the kill zone, to get people out of the danger area, which is the coastline of Texas,” said Johnny Cavazos, Cameron County’s chief emergency director.