South Sudanese rebels accused the army of attacking their positions yesterday, hours before a ceasefire deal agreed by the government and rebels was due to come into effect.
Rebel spokesman Lul Ruai Koang said "simultaneous attacks have been launched" by the army on rebel positions in the northern oil state of Unity, and in the eastern Jonglei region.
But army spokesman Philip Aguer said he had "no reports of fighting", and that clashes in Jonglei had taken place before the deal was signed when rebels attacked government forces.
Government and rebels pledged on Thursday to halt fighting within 24 hours and end five weeks of bitter conflict that has left thousands dead, but both sides have said they doubt the other can fully control the forces on the ground.
Koang alleged that South Sudanese government troops - as well as Ugandan soldiers and rebels from neighbouring Sudan's war-torn Darfur region, the Justice and Equality Movement - had attacked rebel positions, warning that rebel fighters had the "right to defend themselves against this senseless aggression".
The ceasefire agreement was signed late on Thursday in the Ethiopian capital of Addis Ababa by representatives of South Sudan's President Salva Kiir and rebel delegates loyal to ousted vice president Riek Machar, and was cheered by regional peace brokers and diplomats.
US President Barack Obama, whose country provided crucial backing along South Sudan's path to statehood, described the deal as "a critical first step toward building a lasting peace".
UN chief Ban Ki-moon praised mediation efforts and called on both sides to "immediately implement" the agreement.
Mediators from the East African regional bloc IGAD, which has been brokering the peace talks, said the deal would put in place a verification and monitoring mechanism for the truce and allow unrestricted access to aid workers.
Up to 10,000 people are believed to have died in the fighting which has pitted forces loyal to Kiir against a loose coalition of army defectors and ethnic militia nominally headed by Machar, a veteran guerrilla.
Presidential spokesman Ateny Wek Ateny insisted the situation was calm yesterday, and said that orders had already been issued to soldiers "not to fight anybody from now on".
"As far as I know there is no fighting since yesterday ," Ateny told reporters in Juba.
"We hope the rebels will honour their signature."
Government delegation head Nhial Deng Nhial said he was sceptical of the rebels' capacity to rein-in their forces, "given that the bulk of the rebels are made up of civilians" - essentially any one of the countless people who have kept hold of their guns after decades of conflict.
But Rebecca Garang - the widow of South Sudan's independence leader John Garang - who stood with Machar to make a political challenge to the government in December before fighting began, dismissed concerns.
"The ceasefire did not just come alone in isolation without Dr Riek [Machar] knowing what is going on ... so I think he will be able to talk to all of them and control them," she told the BBC.