Liberian authorities have put pro-government forces around the capital Monrovia on high alert.
Government officials accuse rebels of violating the cease-fire near the border with Guinea and at Klay Junction, about 40 kilometers northwest of Monrovia.
Rebels say they have been pushing back government attacks on their positions in recent days. The rebels control most of Liberia, but they have been unable to capture Monrovia during their four-year insurgency.
The cease-fire agreed to at peace talks in Ghana last month calls on Liberian President Charles Taylor to step down and give way to a transitional government.
Mr. Taylor has said he wants international peacekeepers to arrive before he leaves power, while the rebels say he must leave Liberia immediately.
They accuse Mr. Taylor of scheming to turn any multi-national force into what they call a life-support system for his government.
Mr. Taylor also faces an indictment issued by an international war crimes court in Sierra Leone, and regional negotiations are in progress to grant Mr. Taylor safe haven in Nigeria.
Meanwhile, a U.S. military reconnaissance team has been touring Monrovia and other battle-scarred areas in Liberia since last week to assess the possible deployment of U.S. soldiers along with a regional peacekeeping force expected in the coming weeks.