U.S. Ambassador John Blaney and rebel chief of staff Abdullah Sherrif shook hands in the center of a bridge marking the front-line of the war-divided capital, signaling the rebel handover.
Small numbers of West African peace forces and U.S. Marines crossed into rebel territory after the ceremony. Other West African peacekeepers held back tens of thousands of civilians massed on both sides of the New Bridge.
At least four West African military vehicles went directly to the port, control of which is crucial to getting food and other aid flowing again, particularly to the famished government-held side of the capital.
No rebel soldiers could be seen from the bridge. It was not immediately clear if the insurgents had complied with their pledge to pull out of the city entirely by noon Thursday.
The Pentagon expanded the U.S. military presence in Liberia by adding a "quick reaction" force of 150 combat troops to back up Nigerian peacekeepers, even if only for several days.
An additional 50 or so U.S. troops will go as well to provide other help, such as assessing the need for improvements at the port in Monrovia to be used by ships laden with food and other humanitarian supplies.
The first of the expanded contingent of U.S. troops arrived in nine helicopters at Liberia's main airport early Thursday.
Administration officials on Wednesday stressed that the 200 U.S. troops — which are in addition to roughly 100 already there as liaison with the Nigerians and as security for the U.S. Embassy — will not directly do peacekeeping duty and are not intended to become engaged in combat.
President Bush, speaking at his ranch in Texas, emphasized that the Nigerians are in charge.
"They are in the lead, and we are there in support," Mr. Bush said. "My focus now is on making sure humanitarian relief gets to the people who are suffering in Liberia."
Air Force Lt. Gen. Norton Days, director of operations for the Joint Chiefs of Staff, said the troops won't see combat except in an emergency.
But these officials, discussing the situation on grounds of anonymity, also said Thursday the deployment of 200 — after the administration had been saying for weeks that it would send a much smaller group — was meant to step up the pace of African stabilization and humanitarian operations now that President Charles Taylor has left. His departure and rebels withdrawing from the Monrovia port were conditions for U.S. intervention.
There already are about 750 Nigerian peacekeepers on the scene, and the expectation is that once another group of similar size joins them, they will have sufficient strength to stabilize the situation.
At that point — perhaps as soon as next week — the U.S. quick-reaction force would move back aboard Navy ships off the coast, officials said.
In addition to the Marines in the quick-reaction force, smaller numbers of U.S. troops will perform other missions:
About a dozen Navy SEALs are searching the waters of the Monrovia port to clear any obstacles to the arrival of ships carrying food aid. Military engineers will assess the port facilities to determine what improvements are required.
U.S. Harrier fighter jets or Huey or Cobra helicopters will provide what Schwartz described as "aerial escort" as the Nigerian troops head from the airport to the port at Monrovia.
Rebels' control of the port has paralyzed the capital, trapping pockets of desperate need on either side of key bridges. On the rebel side there was food but no doctors, and on the government side, medical are and hunger existed side-by-side.
Hunger has been so widespread in the capital that reports have had Liberians eating leaves, roots and wild animals. Doctors Without Borders (Medecins Sans Frontiers) reports treating more than 700 cholera patients.
The pullback could ease the crisis, but does not signal the end of the war.
On Wednesday, government forces and fighters from Liberia's smaller rebel movement battled on the road leading from Monrovia to the southeastern port of Buchanan. West African peacekeepers and others said clashes were at least 60 miles from the capital.