Rebels in south Sudan's oil-rich Unity state continued for a third day to attack southern army positions, both militia and military officials said on Thursday.

"The rebels attacked the settlement of Mankien early this morning (Thursday)," said Philip Aguer, spokesman for the south's Sudan People's Liberation Army (SPLA).

"We do not have casualty figures at present as the field commanders said the fighting was still going on," he added.

The fighting took place near key oil pumping areas critical to Sudan's economy, close to the still disputed north-south border, as well as the flashpoint Abyei region.

On Wednesday, the United Nations warned that military tensions and political disputes pose a new threat to Africa's biggest nation, which is due to split in two in July after the south voted overwhelmingly for independence in a January referendum.

Aguer said that the rebels fought alongside Arab Misseryia tribesmen, a cattle herding people from just across the border in north Sudan, who were an important proxy militia for the north during Sudan's two-decade civil war.

Rebels, led by former southern army general Peter Gadet, claimed that they had seized Mankien, in Mayom county, and "destroyed several vehicles" belonging to the SPLA.

"We attacked the settlement, and destroyed several vehicles of the army," said rebel spokesman Bol Gatkouth, a former lawmaker in the south's parliament.

He rejected army claims the Misseryia fought alongside the rebels.

"We are not with the Misseryia. They say this only because the army have sent in large reinforcements but cannot stop us," he said.

"We are in control of large areas, including the oil fields."

Tuesday's clashes prompted the Unity state authorities to order the evacuation of northern oil workers from Unity's oil-producing areas, from which much of Sudan's crude is pumped.

A statement said the northerners were being evacuated "until further notice."

But Unity's information minister, Gideon Gatpan, who signed Tuesday's statement, said its meaning had been misunderstood.

"The oil workers were never ordered to be expelled -- only we asked them to be evacuated from certain small areas where they might be in danger for their own safety because of these rebel attacks," he said on Thursday.

"They are welcome and free to work again in all areas once we have ensured it is safe for them," he added.

The fighting began when the militia group, who call themselves the South Sudan Liberation Army (SSLA), blew up two civilian tankers using land mines on Tuesday, before storming an army outpost, killing at least 20 soldiers.

They later abandoned the village before army reinforcements retook the position.

Earlier this month, Gadet's SSLA announced that it planned to overthrow the southern government, denouncing the "rampant corruption" at the top levels of the SPLM, the south's ruling party.

The SPLM has repeatedly accused Khartoum of arming splinter militia groups like Gadet's to destabilise the south ahead of its secession from the north in July, a claim that Khartoum rejects.