Washington - As a massive international relief effort lurches into gear, U.S. officials are stepping up measures to prevent last week's earthquake in Haiti from triggering a Caribbean migration not seen in two nearly two decades.

Experts see no signs for now of a seaborne exodus, although history shows that such events are difficult to predict. That said, South Florida counties have readied contingency plans, immigration authorities have cleared space in a 600-bed detention center in Miami, and Obama administration officials have begun discouraging Haitians from attempting the hazardous 600-mile sea crossing to Florida.

"Please, if any Haitians are watching, there may be an impulse to leave the island and to come here," Homeland Security Secretary Janet Napolitano said Saturday at Homestead Air Reserve Base in Homestead, Fla., where she joined Vice President Biden in speaking to relief workers at a federal staging area.

"This is a very dangerous crossing. Lives are lost every time people try to make this crossing," Napolitano said, adding that Haitians caught at sea would be repatriated. "Please do not have us divert our necessary rescue and relief efforts that are going into Haiti by trying to leave at this point."

The warnings come paired with a giant humanitarian operation to rebuild Port-au-Prince, position U.S. military assets in the area, and adjust U.S. and international immigration policies. In the long run, such measures would be the most effective tools to prevent a refugee crisis, current and former government officials said.

With as many as 3 million people affected by Tuesday's quake, the stakes for the Obama administration in heading off a wave of migrants are high.

"We don't want to have destabilization in Haiti, and deaths of this magnitude in Haiti cause destabilization and have political implications," said Andrew Natsios, a veteran foreign aid official who led U.S. relief efforts in Iraq, Afghanistan and Sudan, and after the Asian tsunami in 2004.

"You cannot prohibit people from moving under international law if they feel threatened. But you can create incentives to stay," said Natsios, head of U.S. Agency for International Development from 2001 to 2006. "If you do the relief response right, most people will want to rebuild."

American presidents since Jimmy Carter have grappled with flows of migrants from the Caribbean triggered by political crises, wars and natural disasters. The sudden influx of 100,000 Cubans and 25,000 Haitians in the 1980 Mariel boatlift left some refugees in U.S. camps for years.

After a coup in Haiti in 1991, the U.S. government housed 12,000 Haitian migrants in Guantanamo Bay and admitted 10,000 before a new government was in place in 1994, a year in which the U.S. Coast Guard stopped and rescued 64,000 Haitians and Cubans at sea.

Spurred by reports that Cuban leader Fidel Castro was in failing health in 2006, the U.S. government embarked on an intensive contingency plan, called Operation Vigilant Sentry, signed by Michael Chertoff, then homeland security secretary, in 2007.

A central feature of the plan is a public messaging campaign designed to dissuade Cubans - then the focus - from risking the life-threatening 90-mile journey across the Florida straits. While electronic mass media in Haiti is virtually non-existent - and the trip to America a harrowing three days instead of few hours - the enormous pending flow of aid, troops and official statements is conveying a similar message.

"The best thing we can do is spend as much relief aid as possible in Haiti," Natsios said. "Make sure food arrives promptly. Restart and stimulate Haitian markets. Restore people's lives."

Nations worldwide have pledged $400 million so far to Haiti, the Western Hemisphere's poorest country, led by $100 million by the United States. U.S. officials are also adopting new immigration measures, combining carrots and sticks. On Wednesday, Napolitano suspended deportations of 100,000 to 200,000 Haitians in the country illegally, and on Friday, she announced that those in the country as of Jan. 12 could apply for temporary protected status so they could help send financial aid to their devastated homeland.

Homeland security officials, however, warned that Haitians caught trying to enter the United States illegally will still be detained and deported. Florida officials said contingency plans contemplate housing migrants at the Homestead base if space beyond the federal Krome immigration detention center is needed.