Updated

Islanders went door to door Thursday to warn neighbors of Hurricane Dean, which spun toward the Caribbean packing 100 mph winds and threatens to strengthen into an extremely dangerous storm by next week.

The first hurricane of the Atlantic season is expected to pass over St. Lucia and the rest of the Lesser Antilles early Friday, then intensify as it enters the warm waters of the Caribbean. It was too early to tell whether the storm would eventually strike the United States.

Click here to track Hurricane Dean.

St. Lucia's acting prime minister, Stephenson King, announced that the country's two commercial airports were closing Thursday night as the storm's outer bands began moving through the islands. Martinique's main airport was also closed.

"We may not be spared on this occasion as it appears that we are likely to experience the worst," King said.

Walter Lech, a medical resident from Boston vacationing in Dominica, said he learned about the hurricane Wednesday when he saw islanders pull boats from the water. Lech and his wife could not find flights off the island and plan to ride the storm out in their rented cinderblock house.

"We're going to take the brunt of it full on, but our neighbors here have been through it (before) and they're not all that worried," Lech said.

About 300 American medical students from Dominica's Ross University were stranded at the island's airport Thursday until family members hired private planes, said Dr. Mauricio Gomez, from the UCLA Medical Center in California, whose fiancee was among the students. Most arrived in Puerto Rico to await flights on Friday bound for the United States, Gomez said.

Hotels in Dominica and Martinique, meanwhile, prepared to move tourists from seaside rooms.

At the Jungle Bay Resort & Spa, on Dominica's Atlantic coast, about 18 guests will spend Thursday night in a reinforced steel-and-concrete shelter, hotel spokeswoman Laura Ell said.

"Everyone's very calm but taking it seriously," she said.

Martinique officials set up cots at schoolhouse shelters while residents lined up at gas stations and emptied supermarket shelves.

"It's the first time I've seen this, all our water supply completely gone in less than two hours," said Jean Claude, a supermarket manager.

The government also canceled commemoration events planned for the 152 Martinique residents who died in a plane crash a year ago.

In St. Lucia, radio and television advisories urged people to stock up on canned food and fill their cars with gasoline. Volunteers knocked on doors to make sure people knew about the storm.

The National Hurricane Center in Miami said Dean would likely be a dangerous Category 3 hurricane by the time it reaches the central Caribbean. Forecasters say it is taking a bead on Jamaica and the southern coasts of the Dominican Republic and Haiti, which share the island of Hispaniola, and could strike the islands on Sunday.

As it approaches the Mexican resort town of Cancun, on the Yucatan Peninsula, on Tuesday it could be an extremely dangerous Category 4 hurricane, the hurricane center said.

It predicted storm surge flooding at 2 to 4 feet above normal tide levels near the center of Dean as it passes over the Lesser Antilles and total possible rainfalls of 7 inches in mountainous areas.

Hurricane Dean strengthened to a Category 2 storm Thursday as it moved closer to islands in the eastern Caribbean, forecasters said.

At 2 a.m. EDT, Dean was centered 85 miles southeast of Martinique and 90 miles northwest of Barbados. It had top sustained winds of 100 mph, up from 90 mph earlier in the day.

Hurricane warnings were in effect for the islands of St. Lucia, Martinique, Dominica and Guadeloupe.

Tropical storm warnings have been issued for the U.S. Virgin Islands, Puerto Rico, Antigua and Barbuda, Barbados, Montserrat, St. Kitts and Nevis, Anguilla and St. Maarten, Grenada, St. Vincent and the Grenadines.

A tropical storm watch was issued for the south coast of the Dominican Republic from Cabo Engano to the Haiti border.