JUBA. 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.

"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.

"The capture of Bentiu will be celebrated by the SPLA, but we’ve seen the town change hands 5-6 times since the conflict first erupted.”

Bentiu fell into rebel hands last month, and opposition forces were accused by the United 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.