Diplomats in the city said they expected the Marines to land in Monrovia from three warships, anchored offshore with 2,300 Marines aboard.
Air Force Major-General Norton Schwartz, director of operations for the United States military's joint staff in Washington, said the 200 Marines would be part of a quick reaction force in reserve to help Nigerian peacekeepers if necessary. Some of them may leave the country the same evening, the Pentagon said.
On Wednesday, US helicopters whirred overhead and two Humvee vehicles were seen in the streets. New President Moses Blah said US jets would soon start patrols to help ensure the success of peace efforts in the country.
"They are hungry. We can't stop them taking food that was brought for them and not distributed."
-- Rebel official Sekou Fofana.
Meanwhile, chaos erupted at Monrovia's port as thousands of hungry people stormed in to grab food from warehouses. This happened as rebels were packing up to leave so that the West African peacekeeping force can move in on Thursday.
Men, women and children scrambled through containers and ripped open sacks in a frantic search for any aid stocks still left. Rebels fired shots and lashed with whips to halt the chaos and stop looters running away with bags of cornmeal on their heads.
"They are hungry. We can't stop them taking food that was brought for them and not distributed," said rebel official Sekou Fofana.
A senior United Nations humanitarian official, who arrived to help coordinate relief for the war-wrecked city, said the UN was looking at a two-year plan to rebuild Liberia.
"The situation on the ground is very desperate," the official Carolyn McAskie said. "We are very concerned about the large number of displaced people and those in their homes who have no access to food."
The latest bout of the 14-year civil war has left 2,000 dead. Former President Charles Taylor went into exile on Monday after installing his deputy Moses Blah as the new president. Blah is scheduled to step down in October and make way for fresh elections.