The ceasefire came into effect at 1730 GMT despite rebel accusations that the army had attacked their positions in two separate regions just hours earlier. (Agencies)
"UN Mission in South Sudan says that sporadic fighting took place in parts of the country today," including after the ceasefire, UN deputy spokesman Farhan Haq said.
Rebel spokesman Lul Ruai Koang said before the ceasefire deadline that "simultaneous attacks have been launched" by the army on positions in the northern oil state of Unity, and in the volatile 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.
Both sides pledged on Friday 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 (JEM) -- had attacked rebel positions, warning they had the "right to defend themselves against this senseless aggression."
The ceasefire agreement was signed on Friday in the Ethiopian capital Addis Ababa by representatives of South Sudan's President Salva Kiir and rebel delegates loyal to ousted vice president Riek Machar, and was greeted by cheers from regional peace brokers and diplomats.
"It is critical that both parties implement the cessation of hostilities agreement in full and immediately," UN spokesman said.
US President Barack Obama, whose country provided crucial backing on South Sudan's path to statehood, described the deal as "a critical first step toward building a lasting peace".
Kiir urged those rebels not under Machar's control to also respect the deal. "Now that people have fought, people should come back to their senses and we sit down so that we can resolve this conflict through negotiation," Kiir said in address on Friday.
Up to 10,000 people are believed to have been killed in the fighting pitting forces loyal to Kiir against a loose coalition of army defectors and ethnic militia nominally headed by Machar, a seasoned guerrilla fighter.
The ceasefire came into effect at 1730 GMT despite rebel accusations that the army had attacked their positions in two separate regions just hours earlier.