BY MARIKA LYNCH, PETER ANDREW BOSCH AND MARTIN MERZER

PORT-AU-PRINCE -- Fifty U.S. Marines swept into Haiti's jittery capital Monday to shield the U.S. Embassy as rebels who already drove government forces from half the country exacted revenge on supporters of President Jean-Bertrand Aristide.

With the insurgents threatening to attack Port-au-Prince within a week and vowing to control the entire country within two weeks, scores of Aristide partisans stood guard at flaming barriers on the northern edge of the capital.

The city remained outwardly calm, but some Cabinet ministers -- fearing the worst -- began seeking safe haven in the homes of friends. France advised its citizens to leave.

The 19-day rebellion already has left more than 70 people dead and dozens wounded. Nevertheless, U.S. officials said they did not see signs of widespread boat-building or similar activities by potential refugees.

An exodus of Haitians trying to reach Florida ''would be a disaster for our state and could mean that many people would lose their lives at sea,'' Gov. Jeb Bush said during a visit to Washington, D.C. He said federal agencies were prepared to handle a worsening crisis in Haiti.

And the crisis was worsening.

Looting raged in the northern city of Cap Haitien, captured by the rebels Sunday. At least one Aristide supporter was seized and beaten by insurgents, and at least four houses of Aristide supporters were torched.

NEW GUNFIRE

Gunfire flared anew on the outskirts of the city Monday night amid rebel reports that pro-Aristide forces attempted a counterattack, which the rebels said was repelled. Rebel officers said they had no word on dead or wounded.

On the political front, Aristide's opponents deferred until today their decision on a power-sharing plan previously accepted by Aristide.

The proposal, backed by the United States, France, the Organization of American States and the Caribbean Community, would require Aristide to accept a new prime minister but would allow him to remain in power.

His opponents seemed on the brink of rejecting the deal Monday, holding firm to their main demand: Aristide must leave now. Under pressure from U.S. Secretary of State Colin Powell and others, however, they delayed a decision until today.

OPPOSITION INSISTENT

''One man cannot keep hostage a nation,'' said Andy Apaid, a spokesman for political opponents gathered under the Democratic Platform banner. Aristide ``must resign. We're not negotiating with him.''

The development came despite a last-minute bid to break the deadlock: a promise that the U.S. and other backers of the plan would support ''further political changes'' if Aristide violated the agreement -- a thinly veiled threat to abandon Aristide if he does not carry out promised reforms.

Opposition leaders say Aristide has promised to make similar changes in recent years but never followed through. Critics also assert that Aristide is at the root of violence, saying he armed his civilian supporters to keep a hold on power.

And so, Aristide still faced a tightening vise, squeezed by armed opponents on one side and political opponents on the other.

Those two enemy camps have in common their hatred of Aristide but otherwise are not closely aligned -- suggesting that even if the political opposition accepts a settlement, the rebels who drove police from virtually all of northern and central Haiti since Feb. 5 could fight on.

Meanwhile, the country edged closer to chaos.

In Cap Haitien, insurgents and their supporters searched for alleged supporters of Aristide, taking away one suspected Aristide supporter who later turned up at a hospital to have several cuts stitched up.

''We're going to clean the city of all chimeres, '' rebel Dieusauver Magustin, 26, told The Associated Press in Cap Haitien, using the Creole word for a mythical monster to describe militant government supporters.

Looters followed the rebels, ransacking newly abandoned homes.

Also looted was a U.N. World Food Program warehouse that stored hundreds of tons of lentils.

Supplies of food and fuel to northern Haiti have been largely cut off by the fighting, and many Cap Haitien residents grew increasingly desperate, fighting over sacks of rice and other commodities.

Later in the day, rebel commanders largely succeeded in suppressing the looting.

Along the perimeter of Port-au-Prince, young men with guns and in some cases only rocks guarded barricades constructed with concrete blocks, tires and the burned shells of cars. Some barricades opened during the day to allow passage, but most closed when darkness fell.

At Toussaint Louverture International Airport, just outside the capital, U.S. Marines in combat gear hustled off the U.S. Air Force plane that carried them to Haiti.

SECURITY MEASURES

The State Department said the Marines, members of a Fleet Antiterrorism Security Team from Norfolk, Va., would augment guards at the American Embassy, the ambassador's home and other U.S. buildings in Port-au-Prince.

''We're trying to make sure that we are prepared to provide security for U.S. facilities there,'' said State Department spokesman Richard Boucher, who called the situation in Haiti ''fluid'' and ``evolving.''

He said U.S. Ambassador James B. Foley had requested the so-called FAST team.

The team, part of a special antiterror unit created in 1987, includes 50 Marines, a canine unit with one dog and two handlers, and two counterintelligence officers.

Herald staff writers Trenton Daniel and Nancy San Martin in Port-au-Prince, Jacqueline Charles and Carol Rosenberg in Miami and Frank Davies in Washington contributed to this report.