South Sudan's army has said it will investigate reports of mass graves discovered by the United Nations as the army prepares to retake rebel-held Bor and Bentiu.

"If anyone has found mass graves we will be willing to investigate it," Colonel Philip Aguer, the army spokesman, told Al Jazeera on Tuesday.

"I have not heard about it, some criminals have targeted people, but the army will investigate any cases where people have been killed for their ethnicity," he said.

Earlier on Tuesday, the UN said it has discovered a mass graves 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.

"We have discovered a mass grave in Bentiu, in Unity State, and there are reportedly at least two other mass graves in Juba," she added.

She said it is unclear who was responsible for the killings.

Troops poised

Aguer, the military spokesman, also told Al Jazeera that army forces are stationed seven miles outside Bor, and will soon advance on the city.

Aguer said the the army has taken over surrounding towns that were under Peter Gadet, who had defected to the rebels.

He said rebels were able to take over Bor and Bentiu, as they were already stationed there and defected to join forces loyal to the former vice president.

"The other cities will be retaken within 72 hours," he added.

There is increasing tension at a UN camp in Bor because armed fighters have entered the congested area where the UN says about 17,000 civilians are seeking protection, AP news agency reported on Tuesday.

US and Ethiopia are leading efforts to open peace talks on the 10-day-old crisis.

President Kiir said on Monday that the army will step up the fight against rebels with imminent strikes in Bor and Bentiu.

Fighting spreads to five states

Meanwhile, UN Secretary General Ban Ki-moon warned warring factions on Tuesday that reports of crimes against humanity will be investigated and prosecuted.

Earlier, Ban said he will recommend to the Security Council an increase of 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.

Fighting has spread to half of the country's 10 states, the UN said on Tuesday.

"The world is watching all sides in South Sudan," Ban told reporters ahead of emergency Security Council talks on the crisis.

Source: Al Jazeera and agencies