United Nations chief Ban Ki-moon has warned warring factions in South Sudan that reports of crimes against humanity will be investigated, as eyewitnesses spoke of a wave of brutal ethnic killings.

Ban asked the security council to nearly double the size of the UN mission in the country, which has been hit by more than a week of escalating battles between troops loyal to President Salva Kiir and those backing his rival Riek Machar, a former vice-president who was sacked in July.

With fighting ongoing, badly overstretched UN bases in the capital Juba and across the country have been flooded with at least 45,000 civilians, some of whom have recounted an orchestrated campaign of mass killings and rape by government forces.

The official toll is 500 dead, although the real figure is believed to be far higher, aid workers say. Hundreds of thousands of others have fled to the countryside, prompting warnings of an imminent humanitarian disaster.

Fighting has spread to half of the young nation’s 10 states, the UN said on Tuesday.

Rebel fighters are also reported to have committed atrocities in areas they control, as the oil-rich but impoverished nation, which won independence from Sudan to much fanfare just two years ago, appeared to be sliding deeper into civil war.

“The world is watching all sides in South Sudan,” Ban told reporters ahead of emergency security council talks on the crisis.

“The United Nations will investigate reports of grave human rights violations and crimes against humanity. Those responsible at the senior level will be held personally accountable and face the consequences — even if they claim they had no knowledge of the attacks,” he said.

President Kiir has accused Machar of starting the fighting by attempting a coup, while Machar says the president has exploited tensions within the army to carry out a purge. Rebels loyal to Machar have since seized control of several areas north of Juba.

The unrest has also taken on an ethnic dimension, pitting Kiir’s Dinka tribe against the Nuer tribe, to which Machar belongs.

Speaking from the relative safety of a UN base in Juba, two ethnic Nuer men alleged they were arrested by government soldiers along with an estimated 250 other men, herded into a police station in the capital Juba and then fired on.

“It was horrible, because to survive you had to cover yourself with the bodies of dead people, and… the bodies started to smell really bad. I don’t want to talk much about it,” said one of the men, named Simon, who would only give his first name for fear of reprisals.