South Sudan's military have said that it was planning a major offensive against rebel forces it claimed were led by former deputy president Riek Machar.

The United Nations has warned that the situation in the world's newest nation is falling apart, with hundreds of thousands of civilians at risk.

Fighting has swamped South Sudan for over a week, after President Salva Kiir accused Mr Machar of attempting a coup.

Mr Machar denied the claim but forces the government claims are loyal to him have since seized Jonglei state capital Bor and Bentiu, capital of oil-producing Unity state.

But South Sudan army spokesman Philip Aguer claimed that the government was now on the offensive.

"Our soldiers will regain control of Bor from the forces of the rebellion," he insisted.

International efforts to avert further bloodshed appeared to be fruitless and UN humanitarian co-ordinator Toby Lanzer, who visited Bor on Sunday, said the situation was rapidly deteriorating.

"It would have been difficult a week ago to imagine things would have unravelled to this extent," he said.

"There are hundreds of thousands of South Sudanese who've fled into the bush or back to the villages to get out of harm's way."

He added that, should fighting break out in Bor, UN peacekeepers were unlikely to be in a position to protect the estimated 15,000 civilians seeking shelter at the UN base.

The clashes have left hundreds dead and sent tens of thousands fleeing for protection to UN bases or safer parts of the country.