Government troops in South Sudan were advancing Monday on the flashpoint town of Bor, the last state capital still in rebel hands, the army spokesman said.

"Bor is still in the hands of the rebels but our forces are still moving towards it," Colonel Philip Aguer said.

Bor, situated some 200 kilometres (125 miles) north of Juba and capital of the restive Jonglei State, has already changed hands three times since fighting broke out in South Sudan one month ago.

The fighting is between forces loyal to President Salva Kiir and a loose coalition of army defectors and ethnic militia nominally headed by Riek Machar, a former vice president and seasoned guerilla fighter.

Government troops recaptured the key north oil city of Bentiu last week, but have since grappled with rebel fighters closer to the capital Juba -- with new clashes reportedly taking place just 20 kilometres from Juba on Sunday.

"Salva Kiir sent a very huge force to attack our position. The attacking convoy was destroyed in a two-hour fight," rebel spokesman Lul Ruai Koang said in a statement on the clashes near the capital.

Aguer confirmed that clashes had taken place but there was no immediate independent confirmation of who had got the upper hand.

The rebel force in the region is commanded by Alfred Ladu Gore, a respected fighter from the Juba region and one of several opposition figures on a government wanted list.

The rebels also claim they are close to retaking Malakal, the capital of the biggest oil producing state Upper Nile.

An AFP photographer who was in Malakal on Sunday said that the town was calm but that the remaining residents were huddled in the town centre, too scared to return to their looted homes.

One month of fighting in South Sudan, which only gained independence from Khartoum in 2011, has displaced some 400,000 people, according to the UN.

The world body put the death toll at well in excess of 1,000, although the independent think-tank International Crisis Group says reports from the field indicate the death toll would be closer to 10,000.