South Sudan on Wednesday rejected a UN proposal to send a 4,000-strong regional force to the restive capital of Juba, saying it undermined the young nation's sovereignty.

The US-drafted resolution presented to the Security Council seeks to establish a protection force of African troops authorised to ‘use all necessary means’ to provide security and deter attacks against UN bases in South Sudan.

But South Sudanese government spokesman Michael Makuei said his country rejected the resolution in its current form as it would ‘(turn) South Sudan into a protectorate and this is a situation that we will not accept.’

The draft, which would also extend the current UN mission's mandate until December, would ‘undermine the sovereignty of the Republic of South Sudan,’ Makuei told reporters in Juba.

The head of the East African bloc IGAD, which first proposed the force, had said on Friday it had obtained South Sudan's permission to deploy it.

But Makuei appeared to throw that agreement into doubt, lending credence to fears among diplomats that the government's apparent willingness to participate in the IGAD summit was partly to buy time.

‘The protection force should have been an independent body, not under (UN mission) UNMISS, so that they can perform their functions and duties... which we had agreed upon,’ Makuei said.

IGAD had raised the possibility of deploying an ‘intervention brigade’ with a more aggressive mandate within the UN mission currently present, along the lines of a similar brigade sent to the Democratic Republic of Congo in 2013.

Juba was rocked by several days of heavy fighting in early July between the government forces of President Salva Kiir and those loyal to ex-rebel chief Riek Machar, the latest upsurge in two and a half years of war.

Nearly 300 people died in the violence, and since then 70,000 South Sudanese have fled the country to Uganda, according to new figures released Wednesday by the Norwegian Refugee Council, an independent aid group.

Sporadic clashes have been recorded since the street battles last month, along with reports of rape and looting, adding to the ranks of 1.6 million displaced within the country since civil war broke out in 2013.

The 12,000-strong UN peacekeeping mission in South Sudan, UNMISS, has faced criticism for failing to stem the latest bloodshed or fully protect civilians during the fighting.