Liberia's leading rebel movement agreed Tuesday to lift its siege of the capital and vital port within two days, allowing food to flow to hundreds of thousands of hungry people.

The accord came as a second rebel group launched a push on Monrovia (search), sending refugees fleeing vicious new assaults a day after warlord-turned-president Charles Taylor (search) resigned and went into exile. Witnesses reported machete-wielding fighters were attacking indiscriminately near the airport.

Rebels from the Liberians United for Reconciliation and Democracy, the main insurgent group, agreed to pull back from Monrovia by noon Thursday and surrender control to peacekeepers.

The pledge was secured by U.S. Ambassador John Blaney (search) and military commanders of a West African peacekeeping force and a 2,300-member U.S. Marine expeditionary force in a meeting with rebel leaders in the looted, rubble-strewn and heavily shelled rebel-held side of Monrovia.

Blaney called the accord "extremely important for the delivery of humanitarian relief."

The deal obliges the multinational peacekeeping force to speed up its deployment throughout the city.

In Washington, a senior defense official said a small contingent of additional Marines might go ashore from ships off Liberia's coast if U.S. commanders need them to get humanitarian aid flowing. No decisions have been made to expand the U.S. military presence, the official said on condition of anonymity.

Since landing the first troops on Aug. 4, the peace force has put on the ground fewer than 1,000 of its promised 3,250 troops. Peacekeepers have ventured only occasionally into the city from their temporary base at the main airport.

Liberia's main rebel group has held Monrovia's port and surrounding districts since July 19, cutting off aid and food to refugees and civilians on the government-held side of the city.

Hunger has built in the capital, with markets offering little but leaves. While civilians welcomed Taylor's flight into exile, hunger and the daily search for food left them too preoccupied to celebrate.

On Tuesday, government fighters fired over the heads of hundreds of civilians massed at one of the bridges leading to the rebel-held port, demanding to be allowed to cross in search of food.

"Everybody's hungry. If we don't die from gunfire we'll die of hunger," said 35-year-old former university instructor Sylvester Lumeh. "We have to take a chance."

"Charles Taylor was the obstacle to peace. Now he is gone, and we just want the port to open," said 23-year-old Emmanuel Barcon outside the U.S. Embassy in government-controlled Monrovia.

"A hungry man is an angry man -- and we're hungry," said Solomon Blamco, 25. He threatened to storm the port himself.

Taylor's resignation Monday came after 14 years of conflict that began when he launched Liberia into civil war in 1989. Taylor quit under pressure from the United States, West African leaders and the rebels, and headed into exile in Nigeria.

The rebels, an ill-disciplined collection of AK-47-toting men and boys, had insisted they would leave the port only when Taylor was gone and when peacekeepers were ready to take the port to keep it from falling into government hands.

The United States is known to have landed fewer than 10 Marines, to act as a liaison between the West African force and the United States. Blaney said any stepped-up U.S. presence "has yet to be determined."

The rebels' two-month siege of the port and nearby bridges have left aid and commercial warehouses pillaged, including at least three depots of the World Food Program, which had 10,000 tons of aid at the port.

Rebel official Sekou Fofana confirmed his forces would withdraw, telling reporters, "We did not come and seize the port for any reason except security reasons. There is no reason to remain ourselves at the free port after Taylor has left."

Brig. Gen. Festus Okonkwo of Nigeria, the West African peace force commander, said the government side would be obligated to withdraw its militias from the city as well. However, the written accord with the rebels said nothing about a government pullback.

Meanwhile, new fighting flared Tuesday on Liberia's second front, the southeast, held by a smaller rebel movement.

The Movement for Democracy in Liberia confirmed it was advancing toward Liberia's main airport, a 45-minute drive from Monrovia. Its leaders, pushing north from the southeastern port of Buchanan, claimed they were responding to attacks by Taylor's forces.

Civilians fled through the bush and on the road in pouring rain, running toward Monrovia to escape attacks most blamed on the southern rebels.

"People are coming and killing," said Pauline Johnson, standing in a downpour and clutching an infant to her breast, after running from her home without pausing to gather any possessions.

Johnson and others said rebels were attacking with machetes, killing men, women and children indiscriminately. One man, who said he was afraid to give his name, said militia were entering homes, killing men of fighting age.

They spoke outside Liberia's main airport, and said the attacks were taking place just a few miles beyond.

In Ghana, Tiah Slanger, chairman of the smaller rebel group, confirmed Tuesday's fighting.

"As I'm speaking to you now, we're in touch with our commanders, impressing them to stop fighting," he said.

Okonkwo said the peace force had heard reports of "disturbances" along the road to Buchanan and had deployed some troops along the route.

It was not clear how strong an offensive the fighting represented, or whether it meant the smaller rebel group was try to take a share of power after Taylor.

Two rebel groups had fought for Taylor's overthrow -- the larger, based in the north, and widely alleged to be supported by neighboring Guinea. The smaller rebel group emerged last year, and is believed supported by Ivory Coast.