South Sudan's army has captured the key rebel base of Nasir and the northern oil hub of Bentiu following a major offensive against rebel commander Riek Machar and his forces.
Army spokesman Philip Aguer told Al Jazeera the Sudan People's Liberation Army (SPLA) recaptured the towns of Nasir and Bentiu from Machar, who has been fighting government forces of President Salva Kiir since mid-December.
We are confident that the the army is on the move to recapture all territories
"The SPLA recaptured Bentiu and Nasir, Bentiu is now in the hands of the SPLA and government forces. We are confident that the the army is on the move to recapture all territories."
Nasir, situated close to the border with Ethiopia, has been one of the main bases of Machar and his rebel army.
Government troops also moved into the northern oil hub of Bentiu, capital of the oil-rich Unity State and a town that has changed hands several times throughout the conflict.
Al Jazeera's Anna Cavell reporting from Juba said the capture of Bentiu represented a major victory for the government.
"Battles are incredibly fluid. Towns rise and fall with regularity. Bentiu itself has changed hands five or 6 times recently. So this remains fluid, but for today, it's a big gain for the government forces."
Bentiu fell into rebel hands last month, and opposition forces were accused by the Unitied Nations of massacring hundreds of civilians in the town.
Both sides in the conflict have been accused of war crimes including mass killings, rape, attacks on hospitals and places of worship, and recruiting child soldiers.
The advance by government forces comes just days after President Kiir agreed to hold direct talks with Machar on ending the country's civil war during a visit by US Secretary of State John Kerry.
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Kerry had visited South Sudan on Friday and secured agreement from President Kiir to meet with Machar in Addis Ababa in the coming days, with Ethiopian Prime Minister Hailemariam Desalegn acting as mediator.
The top US diplomat, who brandished the threat of sanctions if either Kiir or Machar fail to end their war, said he hoped the two would agree to finally implement a moribund ceasefire deal and set up a transitional government.
President Barack Obama signed a decree last month authorising punitive sanctions, including the seizure of assets and visa bans, against anyone in South Sudan deemed to be threatening peace efforts.
The conflict in the world's youngest country has claimed thousands of lives, with at least 1.2 million people forced to flee their homes.