Oil production in South Sudan's Unity State has fallen about a quarter since July partly because the withdrawal of Sudanese workers and technology made some wells temporarily inaccessible, an oil official said.

Unity State oil minister William Garjang Gieng said 15 of 138 wells are out of action in Unity and Mid oil fields, causing the state's output to drop to around 80,000-95,000 barrels per day (bpd), down from around 115,000-120,000 bpd.

South Sudan produces about 75 per cent of the formerly united Sudan's roughly 500,000 bpd, although these numbers can vary with conflict, weather and other factors. Sudan's total production was around 450,000 bpd in June, the oil minister at the time said.

Some of the wells could not be accessed during the rainy season because Sudanese staff, materials and road-building equipment pulled out before independence, he said. He expects production to bounce back to normal early next year.

"There are some wells that have problems and they need to be reworked by the service rigs, but they don't have access to the roads because that is a swamp area," Garjang told reporters in Bentiu, the state capital, late on Tuesday.

"The other problem that is facing people is the (amount) of manpower that is working in the area."

South Sudan seceded from Sudan in July following a January referendum, the culmination of a peace deal signed in 2005 that ended decades of civil war. They have yet to agree on issues including how to manage the formerly integrated oil industry.

At independence Khartoum pulled out 1,300 Sudanese oil workers, leaving just 300 foreign workers and 38 Southern Sudanese to run the Unity State oil fields, Garjang said.

Some of the skeleton staff left working in the oil fields are volunteers who have not been paid for five months, he said.

Unity oil field, the largest of three in the state, is run by Greater Nile Petroleum Company, a consortium of national oil companies from China, Malaysia, India and South Sudan.

Sudan and South Sudan accuse each other of supporting rebellions in their shared border territories, including Unity state.

"Khartoum wants to cause any problem that will stop this field from working and that will have a negative impact on the economy in South Sudan," Garjang said.

South Sudan has accused Khartoum of training the insurgent South Sudan Liberation Army (SSLA) and equipping them with landmines and guns, a charge Khartoum denies.

The South accuses the SSLA of laying anti-personnel and anti-tank landmines across Unity State and attacking Mayom town in late October. But Garjang said the rebels did not threaten oil production.

"Those who are in the field are operating because they (the SSLA) are not targeting the oil field, only the government vehicles. (The SSLA) don't want to stop the operation in the field."

Garjang said he wants to install South Sudanese government employees in Heglig, an oil town across the border in Sudan where oil from Unity's three fields is pumped.

"We need to have people who are working there from the government of Southern Sudan to know exactly the quantity of the oil being pumped to final terminal because sometimes we may be cheated."