Peace talks aimed at ending nearly two months of fighting in South Sudan are under threat after rebels refused to take part in the next round of negotiations aimed at ending months of fighting.

An opposition delegation spokesman, Taban Deng, said rebels would not take part in Tuesday's talks until prisoners had been released and foreign forces had withdrawn from the country.

He also called for the immediate withdrawal of Ugandan troops, who have  been in the country at the request of South Sudanese president Salva Kiir since the conflict erupted on December 15.

The second round of talks in Ethiopia was due to open Monday, but was delayed for logistical reasons.

Blocking the talks "would be tantamount to holding hostage the people and the nation because of those demands," Seyoum Mesfin, chief mediator from the regional bloc IGAD, told reporters.

"It contradicts the commitment of their leader who assured the envoys...that he would not put those demands and positions as a condition," he said in Addis Ababa.

Lul Ruai Koang, a spokesman for the rebel group Sudan People's Liberation Movement, said there were three reasons for suspending the talks.

He told Al Jazeera that four political prisoners remained in prison, that Kenyan authorities had prevented seven former prisoners in Nairobi from travelling to Addis Ababa and that the government and its allies had violated the ceasefire.

Source: Al Jazeera and agencies