According to army officials, there have been heavy clashes in the northern town, which is the capital of oil-producing Unity State.
“We are fighting in and around Bentiu to take back control,” media outlets quoted Army spokesman, Philip Aguer, as saying, adding, “They are resisting but we have the upper hand.”
The fighting comes a day after government troops moved to regain control of the violence-wracked region.
Bentiu fell into rebel hands last month, and opposition forces were accused by the United Nations of massacring hundreds of civilians there.
Just a few days ago, President Salva Kiir agreed to hold direct talks with rebel leaders during a visit by US Secretary of State John Kerry.
The conflict in South Sudan has pitted government forces loyal to President Salva Kiir, a Dinka, against rebels aligned with former vice president, Machar, who hails from the Nuer tribe.
Thousands of people have been killed and many others displaced by more than four months of fighting between government troops and rebels.
Mass killings, which appear to be ethnically motivated, have sparked international condemnation.
The UN sources say despite January’s ceasefire, government and opposition forces are still engaged in heavy fighting in several areas including Jonglei, Unity and Upper Nile states.