The United States has expressed deep concern over the recent upsurge in fighting in Liberia.
There are not enough peacekeepers for the entire country
Thousands of civilians have been fleeing a new advance by Lurd rebels in north-eastern Nimba County, a stronghold of former President Charles Taylor and one of the few areas still under government control.
Aid agencies say that several thousand people are leaving the fighting around the town of Gbatala, some 100km north-east of the capital, Monrovia.
The State Department said the renewed violence was jeopardising the peace agreement signed last week and hampering relief efforts.
Spokesman Philip Reeker criticised both government forces and the two main rebel groups, and said threats by Lurd to launch new incursions in northern Liberia were unacceptable.
On Sunday, US marines in Liberia left for warships stationed off the coast. Their departure after only 11 days caused dismay in the capital, Monrovia.
On Tuesday, Liberia's interim president called on West African peacekeepers to deploy outside Monrovia.
There are some 1,500 Nigerian troops in Liberia, with 700 due this week from other West African nations, however a spokesman for the Ecomil force said this was not enough to venture outside Monrovia.
"We want [regional body] Ecowas to hurry up with the deployment and make sure they deploy throughout the country to stop the carnage on our people," President Moses Blah told Reuters news agency.
Over the weekend, there were reports of a massacre in the Nimba town of Bahn.
"They killed some 25 people. I saw the bodies myself," Targen Wanteh, a former Liberian ambassador to Guinea, told Reuters by satellite phone from the bush near Bahn after fleeing what he said was a rebel attack.
"They shot some people, cut women and children into pieces, opened up their stomachs, cut their heads and laid the bodies in front of their houses," Mr Wanteh said.
"Ecomil is sitting in Monrovia and we are here in the bush, surviving on bush yams."
"We're still very thin on the ground. We don't have enough men," said Ecomil spokesman Colonel Theophilus Tawiah, adding that the next target for the force would be to deploy to the rebel-held port city of Buchanan.
A peace deal signed in Ghana a week ago had raised hopes that the 14-year war might be at an end.
Mr Blah is due to hand over power to a power-sharing government led by businessman Gyude Bryant in October.
Lurd rebels have advanced from the town of Gbarnga to Gbatala, according to latest reports.
Gbatala is near three refugee camps housing some 50,000 people and some have started to flee south, towards the relative calm of Monrovia.
But the rebels said government soldiers had mounted assaults on
their positions at a farm in the area once belonging to former president Taylor.