But the regional Igad bloc mediating the talks, due to resume today, said rebel leader Riek Machar had agreed to have his delegation take part in the talks.
The Addis Ababa talks, which secured a ceasefire last month, aim to end a conflict that has claimed the lives of thousands of people and displaced more than half a million South Sudanese since it broke out on December 15.
The rebels said they wanted four remaining political prisoners held by the Juba government to be released and the Ugandan army, which has been backing President Salva Kiir, to withdraw from South Sudan.
"We are abstaining from participating in the next round of peace talks," the rebels said in a statement late yesterday.
But Seyoum Mesfin, chief mediator for the regional Igad bloc leading the negotiations, said the statement was at odds with previous stance and promises from Machar, a former vice president.
"It contradicts the commitment of their leader who assured the envoys... that he would not put those demands and positions as a condition. That would be tantamount to holding hostage the people and the nation because of those demands," Seyoumn told reporters in Addis Ababa.
The United States expressed concern on Saturday over reported violations of a shaky ceasefire agreement between South Sudan's government and rebel forces and urged the removal of foreign fighters engaged in the conflict.
The US has also previously requested for the South Sudan government to release the last four of a group of 11 detainees held on suspicion of trying to stage a coup.
Seyoum said the talks have been delayed by a day so that seven freed political prisoners – out of the 11 originally arrested – can travel to Addis Ababa to take part in the talks.
However, Seyoum added that the former prisoners did not wish to side with the rebels or the government and might instead form a "third block" in the talks.
"They rather would like us to prepare a sort of a triangular table for their negotiations," Seyoum added.
The rebels in their statement said they had evidence that the Ugandan army was "still actively engaged in combat" despite a ceasefire, and accused Kiir's forces of besieging a United Nations compound housing displaced people in Juba.
Both sides have in the past accused each other of wanton killings and ethnic-based reprisals. – Reuters, February 11, 2014.