United Nations peacekeepers are due to fly back to Liberia's Nimba County after finding evidence of fighting.
Peacekeepers have begun deploying outside Monrovia
They were unable to land on Sunday but saw armed fighters in the streets and "huge columns of smoke" hanging over some towns, officials say.
The government and Model rebels accuse each other of starting the attacks despite an August peace deal.
Former President Charles Taylor launched his rebellion in Nimba and it is home to many of his fighters.
A UN official told the BBC's Jonathan Paye-Layeleh in the capital, Monrovia, that they were unable to identify which group was responsible for the fighting.
In Monrovia, the deployment of peacekeepers and the installation of an interim power-sharing government has brought calm.
But elsewhere, armed gangs of both Taylor loyalists and rebels continue to kill, rape and loot.
Defence Minister Daniel Chea, a Taylor loyalist, said that at least
two towns had been over-run by the rebels.
A Nimba county citizen now residing in Monrovia but in phone contact with government commanders in the region, Harrison Karnweah, told our correspondent that "my information is that the towns of Gbanquoi and Grei have been attacked and occupied by the rebels since
The two towns are situated on the route leading to the border with the Ivory Coast.
Model, one of two Liberian rebel groups that signed a truce with the
government of former President Taylor, operates in the border region, our correspondent says.