Liberia Broadcasting System said the massacre was carried out at the town of Bahn, in Nimba County, on Sunday and blamed the rebel faction known as MODEL.
The report did not give specific reasons or details for the killings and there has been no independent confirmation of a massacre.
But the radio station quoted a VHF radio operator in the region as saying "there were about 1000 people killed in this massacre".
Liberia's government and rebels have accused each other of violating a peace deal signed since the departure of former president Charles Taylor earlier this month.
His departure was meant to help an end to nearly 14 years of violence in the West African country.
Rival factions and government forces have accused each other of attacks around the second city of Buchanan, southeast of Monrovia, as well as fighting in the northeast.
Fleeing the fighting
Thousands of civilians fled what appeared to be fresh fighting near Liberia's second-largest city of Buchanan on Saturday as caretaker President Moses Blah sought to cement a new peace deal.
Men carrying household possessions and women with babies strapped to their backs streamed along the main road leading from the rebel-held port city to the West African country's capital, Monrovia.
The reports of fighting underlined the fragility of a peace deal signed earlier this week by Blah's government and two rebel groups - the Liberians United for Reconciliation and Democracy, or LURD, and a smaller faction, Movement for Democracy in Liberia, or MODEL.
It was not immediately clear who was fighting whom, but some refugees said MODEL fighters were trying to advance on Monrovia, about 60 miles from Buchanan.
But West African peacekeepers said they were told the shooting was carried out by government militias.