South Sudan's army backed by helicopter gunships were advancing Saturday towards the rebel-held town of Bor, a spokesman said, three days after it was wrested from them.

"We are moving towards Bor... there is fighting, but we are supported by air units," Philip Aguer, the spokesman for the Sudan People's Liberation Army (SPLA), told AFP.

Fighting broke out last Sunday between troops backing President Salva Kiir and those of his former vice president, Riek Machar.

Troops loyal to the fugitive Machar seized the town of Bor, which lies some 200 kilometres (125 miles) north of Juba late on Wednesday,

Bor is the capital of the eastern state of Jonglei, one of the most volatile regions in the young nation

Aguer dismissed reports that neighbouring Uganda's army -- which sent in troops to the capital Juba on Friday to help secure the city and support the evacuation of thousands of their citizens -- were involved in the battles.

"This is an operation of the SPLA alone," Aguer said.

He also insisted that the key oil-producing Unity state remained under government control, reporting fighting in the state capital Bentiu overnight.

"There was shooting in Bentiu overnight Friday to Saturday... We are awaiting exact details on the nature and extent of that," Aguer added.

"But I can be clear, Unity state is still under SPLA control."

At least five South Sudanese workers were killed late on Wednesday after unidentified attackers stormed their compound in Unity state.

Oil production accounts for more than 95 percent of South Sudan's fledgling economy, with the campaign group Global Witness warning that "if rebel forces were to capture the oil fields, they could effectively hold the government to ransom."

Chinese state oil company China National Petroleum Corp (CNPC) said Friday it was evacuating workers because of violence in South Sudan that has left hundreds dead.

Neighbouring Sudan has also warned it is concerned over the fate of vital oil flows as fighting between rival army factions spread in neighbouring South Sudan.