South Sudan's warring forces have boycotted a new round of peace talks to protest against being accused of seeking a military solution to the conflict, mediators say.

The latest blow to peace efforts comes after several months of talks that have already been hit by delays and boycotts over other seemingly trivial issues, such as the venue of the talks themselves.

"The two parties, who are not here with us, told us that they are not joining us," chief mediator Seyoum Mesfin, from the East African regional bloc Inter-Governmental Authority on Development (IGAD), told reporters on Monday.

South Sudan's government said it objected to comments made last week by IGAD executive secretary Mahboub Maalim that the two sides were "stupid" for believing military victory was possible.

Government team leader and Minister of Information Michael Makuei told AFP news agency they would "suspend attendance or participation in the negotiations until this issue is addressed".

Talks cost $17m

The rebels said they were boycotting talks, claiming they were not consulted on who would attend, calling the selection of delegates "faulty" and demanding a "transparent and inclusive process".

The talks, being held in the Ethiopian capital Addis Ababa, have so far cost over $17m but have failed to stop the war.

Mesfin pressed both sides to return to the negotiating table urgently.

"With every day passing, human lives are lost and destruction continues," he said.

President Salva Kiir and his arch rival Riek Machar committed themselves again last week to a ceasefire and agreed to forge a transitional government within 60 days. Two previous ceasefire deals swiftly collapsed.

Fighting broke out on December 15, pitting government troops against militia forces loosely loyal to Machar. The violence has taken on an ethnic dimension, with Kiir's Dinka people fighting the Nuer, Machar's tribe.