South Sudan's President Salva Kiir has said that government troops have taken control of previously rebel-held Jonglei state capital Bor.
"Forces loyal to the government have taken Bor and (are) now clearing whatever forces that are remaining there," Kiir told journalists at his office in Juba.
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South Sudan's military spokesman Philip Aguer told Al Jazeera on Tuesday that the army was advancing on Bor and that "the other cities will be retaken within 72 hours."
He said that army forces were stationed seven miles outside Bor, and would soon advance on the city.
The AP news agency also reported that there was increasing tension at a UN camp in Bor because armed fighters entered the congested area where the UN said about 17,000 civilians are seeking protection.
Earlier in the day, South Sudan's army said it will investigate reports of mass graves discovered by the United Nations.
"If anyone has found mass graves we will be willing to investigate it," Colonel Philip Aguer, the army spokesman, told Al Jazeera on Tuesday.
Earlier on Tuesday, the UN said it has discovered a mass grave containing about 75 bodies in Bentiu in the oil-rich Unity state.
"Mass extrajudicial killings, the targeting of individuals on the basis of their ethnicity and arbitrary detentions have been documented in recent days," Navi Pillay, UN High Commissioner for Human Rights said in a statement.
She said it is unclear who was responsible for the killings.
UN Secretary General Ban Ki-moon said he will recommend to the Security Council to increase the number of peacekeepers from 5,000 to about 11,800 to better protect civilians.
The official death toll from the fighting stands at 500, although the real figure is believed to be far higher, aid workers said.
Hundreds of thousands have fled to the countryside, prompting warnings of an imminent humanitarian disaster.