MONROVIA, Liberia - Dozens of American troops landed at Liberia's main airport today, increasing the U.S. presence to boost West African peacekeepers, as rebels began withdrawing from Monrovia, ending their two-months siege of the starving capital.

U.S. Ambassador John Blaney and rebel chief of staff Abdullah Sherrif shook hands in the center of a bridge marking the frontline of the war-divided capital, signaling the rebel handover.

The rebel withdrawal opens access to Monrovia's vital seaport and allows food and supplies to flow again, particularly to the famished government-held side of the capital, where hundreds of thousands of residents and refugees have had little more than leaves to survive on.

Small numbers of West African peace forces and U.S. Marines crossed into rebel territory after the ceremony, and at least four West African military vehicles went directly to the port.

Firing into the air, insurgents left the port and retreated north, heading toward the Po River, their promised new boundary outside the city. They kept their AK-47s, rocket-launchers and other arms, and many carried away stereos, sacks of food aid and other loot.

Tens of thousands of civilians massed on both sides of the New Bridge. West African peacekeepers, trying to curb the chaos, held back hungry crowds on the government side as well as civilians on the former rebel side, who rushed the bridge by the thousands, shouting, "We want peace!"

Excited masses on both sides cheered as two U.S. fighter jets swooped back and forth above the crowd.

Rebels had promised to pull out of the areas of the capital they control by noon today, following the resignation and departure on Monday of President Charles Taylor, a former warlord blamed for 14 years of conflict in Liberia.

President Bush had refused to send in any significant number of troops until Taylor left - and today's deployment dramatically increased the number of U.S. troops on the ground in Liberia, from only about a dozen to a planned 200, including a 150-member rapid reaction force.

U.S. Marines armed with M-16s and wearing helmets and jungle camouflage jumped out as nine U.S. helicopters settled on the airport tarmac, with two more helicopters hovering overhead.

"We are just here to help the people," Sgt. Michael Hobbs said minutes after arriving. All the 200 additional U.S. soldiers were expected to arrive today.

"This operation today is going to be an important one," said Blaney, the U.S. ambassador, meeting the arrivals at the airport. "You are going to see American boots on the ground, and a firm commitment to uphold humanitarian concerns in this country."

Fifty members of the new U.S. deployment are to help with the logistics of getting aid flowing again to Liberia's cut-off capital.

Taylor's exit has raised hopes for an end to the bloodshed. Fighting since 1989, when Taylor launched the country into civil war, has killed more than 100,000 people, left the once-prosperous country in ruins, and left the population prey to armed fighters on both sides who loot and rob.

Washington has stressed that the U.S. role would be as back-up to African peace troops, and that they would be concerned primarily with getting in humanitarian supplies. It said it did not intend the Americans to take part in combat.

For Liberians, sight of the American helicopters and troops was enough.

"I am so happy. All these years we've been praying for America to come," said Randolph Eggley, a 51-year-old worker at the airport. "Today maybe peace will begin."

The troops, members of the 26th Expeditionary Unit, have been waiting in three U.S. warships off Liberia and have now pulled to within view of the city.

Securing the port is a key first step for peacekeepers. Heavy looting of U.S. and World Food Program aid warehouses and other stockpiles of food continued this morning ahead of the rebels' withdrawal. Thousands of people poured out of the area around the port with bags of grain and other goods on their heads. Rebels fired into the air as looters fought each other for the booty.

West African nations have been landing peacekeeping troops since Aug. 4, keeping them at a temporary base at the airport until the force reached sufficient strength to deploy in the capital. Monrovia, a city of more than 1 million, is now crowded with hundreds of thousands of refugees.

About 800 West African peacekeepers, mostly Nigerian soldiers, have landed so far, and a second Nigerian battalion was to start flying in later today.

Leaders of Liberia's post-Taylor government said they welcomed the U.S. deployment. "It's long-awaited and we thank God it's been realized," said Lewis Brown, the foreign minister. It "leads one to believe we might be closer to the end."

Brown spoke at the airport as he accompanied new President Moses Blah to Ghana and ongoing peace talks there.

Brown said Blah would meet with leaders of both of Liberia's rebel groups for talks that would focus on their demands that Blah step down.

West African leaders say Blah is to hand over power in October to a transitional government that will lead Liberia to new elections. Rebels insist Blah is only a stand-in for Taylor, and want him out sooner.

A cease-fire has held in Monrovia since the West African peace troops first deployed. Fighting has persisted outside the capital, with recent clashes to the south as Liberia's second rebel group moved forward from the country's second-largest city, the southeastern port of Buchanan.

Blaney, the U.S. ambassador, said the southern rebels have now agreed to withdraw to within a few miles of Buchanan, using the St. John River as a cease-fire line.