Many people have been killed and villages set on fire in a town in north-east Liberia, according to reports.
Liberians were dismayed by the departure of the US troops
Liberia Broadcasting System said the attacks in Bahn, Nimba County were carried out by rebels from the Movement for Democracy in Liberia (Model).
"My understanding is that there was a massacre but we are not
exactly sure how many people have been killed, it could be
100 it could be 1,000," General Benjamin Yeaten, deputy head of the government army, told the AFP news agency.
A peace deal signed in Ghana a week ago had raised hopes that the 14-year war might be at an end.
Nimba County was one of the last areas to remain under government control and was one of former President Charles Taylor's strongholds in the 1990s civil war.
A source quoted by Liberia Broadcasting System also said about 1,000 people had been killed in the Bahn attacks but there has been no independent confirmation of the deaths nor information on when they took place.
Liberian Defence Minister Daniel Chea said he knew nothing about the incident.
Although a three-week-old West African peace-keeping force has helped stop
fighting in the Liberian capital, Monrovia, clashes have persisted in the countryside.
The government and rebel groups have accused each other of provoking the fighting.
Mr Chea said rebels from the main rebel group Lurd had carried out an attack near the central city of Gbarnga, around 160 kilometres (100 miles) north-west of Monrovia.
The rebels said government soldiers had mounted assaults on
their positions at a farm in the area once belonging to former President Charles Taylor.
Fighting was also reported near Harbel, 65 km (40 miles) from the capital, by UN staff which sent thousands of people fleeing.
On Sunday, US marines in Liberia left for warships stationed off the coast.
A spokesman for the US military, Lieutenant Colonel Tom Collins, said the 150 marines from the 250-strong contingent in Liberia would wait on the ships in case they were needed again.
But the marines' departure after only 11 days caused dismay in Monrovia.
"They're forsaking us," said Emmanuel Slawon, 22, as he watched the US helicopters leaving the city's airport.
"We wish they'd stay until peace would come - Their presence here puts fear in our fighters."
Caretaker President Moses Blah has been touring states in the region
to cement the peace deal.