Between 20,000 to 30,000 local people have fled the fighting.
Liberian Defence Minister Daniel Chea, just back from the frontline, accuses Guinea of backing the rebels.
He said the seizure of the town amounted to "an overt declaration of war" by Conakry.
There has been no independent confirmation that the provincial capital of Lofa County has fallen, but Voinjama has changed hands at least once in the past year.
Rebels held Voinjama from approximately July to October 2000.
Liberia and Guinea routinely accuse each other of supporting each other's rebels.
More than 100,000 refugees, mainly from Sierra Leone and Liberia, are currently trapped by fighting in the region of southern Guinea just over the border from Voinjama.
Advance
Reports suggested that government troops have now secured some villages surrounding Voinjama.
Between 20,000 to 30,000 local people have fled the fighting.
According to the government, pick-up trucks have been transporting weapons to the rebels across the border from Guinea every day
Our correspondent in Liberia says that there are hints from defence sources that a one-time "strongman" of the late President Samuel Doe regime could be behind the latest rebel attack
General Julu staged a coup in September 1994 when he briefly occupied the Executive Mansion in Monrovia, but was kicked out by the West African Peacekeeping Force, Ecomog