Liberian rebels and government forces are waging fierce battles for control of the eastern town of Gbarnga.
There are reports the rebels may have seized the strategic Saint Paul bridge near Gbarnga.
The U.N. office of humanitarian affairs reports the upsurge in fighting has caused 15,000 people to flee the area. The U.N. agency says there are now 75,000 internally displaced people in northeast Liberia.
Many have headed to Kakata town, north of the capital, Monrovia.
Last year, the rebel Liberians United for Reconciliation and Democracy seized Gbarnga, but it was retaken by the government. It used to be the headquarters of former rebel leader, and current president, Charles Taylor.
The rebels have been trying to oust the Taylor government since 2000.