He also described as baseless a government allegation that forces loyal to Mr Machar had damaged oil facilities there.
In another development, South Sudan's Oil Minister, Stephen Dhieu Dau, visited Khartoum to discuss the impact of the conflict on the oil industry with his Sudanese counterpart, Makawi Mohammed Awad.
South Sudan seceded from Sudan in 2011 after a long and bloody conflict, to become the world's newest state.
Most of the oilfields are in the South but the pipelines run through Sudan and oil is a crucial source of revenue for both countries.
The UN Security Council has urged South Sudan President Salva Kiir to release the political prisoners.
However, Mr Machar's forces appear on the back foot after losing the town of Bentiu, the capital of oil-rich Unity State, to government forces on Friday.
The government says it is mobilising thousands of troops to retake Bor - the last major town controlled by Mr Machar's forces.
The conflict began on 15 December between forces loyal to the president and forces loyal to Mr Machar, his former vice-president.
According to the UN, the fighting has killed "very substantially in excess" of 1,000 people.
Fighting erupted in the South Sudan capital, Juba, in mid-December. It followed a political power struggle between President Salva Kiir and his ex-deputy Riek Machar. The squabble has taken on an ethnic dimension as politicians' political bases are often ethnic.
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
Both Sudan and the South are reliant on oil revenue, which accounts for 98% of South Sudan's budget. They have fiercely disagreed over how to divide the oil wealth of the former united state - at one time production was shutdown for more than a year. Some 75% of the oil lies in the South but all the pipelines run north
The two Sudans are very different geographically. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest.
After gaining independence in 2011, South Sudan is the world's newest country - and one of its poorest. Figures from 2010 show some 69% of households now have access to clean water - up from 48% in 2006. However, just 2% of households have water on the premises.
Just 29% of children attend primary school in South Sudan - however this is also an improvement on the 16% recorded in 2006. About 32% of primary-age boys attend, while just 25% of girls do. Overall, 64% of children who begin primary school reach the last grade.
Almost 28% of children under the age of five in South Sudan are moderately or severely underweight - this compares with the 33% recorded in 2006. Unity state has the highest proportion of children suffering malnourishment (46%), while Central Equatoria has the lowest (17%).
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