According to military sources, fighting broke out on Monday involving rebels of the Movement for Democracy in Liberia (MODEL).
"This fighting has led to the burning of towns and homes in areas like Gray, Zuatuo and other towns," an official, who declined to be named, said. "This is very serious and we have informed the United Nations about this incident."
The renewed fighting has highlighted the challenges of bringing peace to the country's unpredictable interior where armed fighters roam – far from United Nations peacekeepers deployed in the coastal capital of Monrovia.
"We have received information about fighting in that area and we are concerned about this. On Sunday, a team went to Nimba in a helicopter to find out what the situation was like," a UN official, who declined to be named, said.
"We saw some areas on fire, but we will go there later today," the official said.
No deaths have been reported so far.
Liberia, a nation founded more than 150 years ago by freed American slaves, has been haunted by 14 years of civil war.
The resignation of President Charles Taylor brought jubilation and increased hope for a lasting peace this past September.