Government troops in South Sudan have been advancing on the flashpoint town of Bor, the last state capital still in rebel hands, according to an army spokesperson.

Bor, situated about 200km 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.

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

The conflict in South Sudan has raged for weeks 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 northern oil city of Bentiu last week, but have since grappled with rebel fighters closer to the capital Juba. The rebels also claim they are close to retaking Malakal, the capital of the biggest oil-producing state, Upper Nile.

Ceasefire talks

Meanwhile, mediators were meeting separately with negotiators from both sides of the South Sudan conflict in an effort to reach a ceasefire deal.

The Intergovernmental Authority on Development (IGAD) said mediators needed to minimise the number of conditions that parties wanted fulfilled before they could commit to signing a ceasefire agreement.

Until now, the main obstacle has been the fate of political detainees being held in Juba, but other conditions include the withdrawal of Ugandan forces allegedly deployed to help Kiir and the lifting of the state of emergency imposed on Unity and Jonglei states.

The rebels are also demanding the withdrawal of fighters from the Darfuri Justice and Equality Movement (JEM), who they say are aiding government forces.

"They aren't calling it a deadlock yet but that seems to be where these talks are headed," Al Jazeera's Mohammed Adow said.

One month of fighting in South Sudan, which gained independence from Khartoum in 2011, has killed more than 1,000 people and displaced about 400,000, according to the UN.