South Sudan's rebels said Sunday they did not expect the government to respect a truce and agree to a peace plan, despite renewed pledges to end an 11-month civil war.
Arch-rivals President Salva Kiir and rebel leader Rief Machar ended two days of talks in the Ethiopian capital Addis Ababa on Saturday and vowed an "unconditional, complete and immediate end to all hostilities."
Hours later, however, the rebels accused government forces of attacking their positions near the oil-rich northern town of Bentiu.
"To be honest, I'm not confident," rebel military spokesman Lul Ruai Koang told reporters when asked whether he believed the government would respect the truce.
"The problem in the past was that we have been dealing with an unfaithful partner. This new agreement is going to be a litmus test for the government forces to know whether they are serious or wasting time," he said.
South Sudan's Information Minister Michael Makuei insisted the government was committed to ending the war, which has left tens of thousands dead, forced almost two million from their homes and pushed the world's youngest nation to the brink of famine.
"The cessation of hostilities will begin to be operational today," he said after the two sides signed off on a detailed plan to pull back their forces, protect civilians, and allow humanitarian access and ceasefire monitoring.
"I don't know about the other side, but on our side we are very committed to the cessation of hostilities," Makuei said.
The East African regional bloc IGAD, which has been trying to broker a peace deal, has given the pair just 15 days to finalise a transitional power-sharing accord.
Kiir and Machar signed a ceasefire at the start of the year and several subsequent deals to renew it, but the truces have been short-lived.
IGAD, which groups Djibouti, Ethiopia, Kenya, Somalia, South Sudan, Sudan and Uganda, has warned that yet another violation would result in sanctions including asset freezes, travel bans and an arms embargo.
The bloc has also threatened to "take the necessary measures to directly intervene in South Sudan to protect life and restore peace."
Fighting started in December last year, when Kiir accused his sacked deputy Machar of trying to stage a coup, with the violence broadening into an ethnic conflict and now including more than 20 different armed groups.
Recent weeks have seen an upsurge in fighting coinciding with the end of the rainy season, and there have been heavy clashes in several areas.
South Sudan, which gained independence from Khartoum in 2011, is the world's youngest nation.