A ceasefire between followers of President Salva Kiir and his former vice-president Riek Machar took effect at 5:30pm GMT.
"The United Nations [UN] mission in South Sudan says that sporadic fighting took place in parts of the country today [Friday]," including after the ceasefire, UN deputy spokesperson Farhan Haq said.
South Sudanese rebels accused Kiir's army of attacking their positions ahead of the ceasefire, which was brokered by East African nations and agreed to on Thursday in Addis Ababa. The government army said it knew nothing of any fighting since the deal was reached.
"It is critical that both parties implement the cessation of hostilities agreement in full and immediately," the UN spokesperson said.
The Kiir and Machar forces are still battling for several towns including Jonglei state capital Bor and Upper Nile state capital Malakal, where there has been bitter fighting.
"It won't happen overnight when there was so much fighting going on," said one UN Security Council diplomat following the crisis.
Haq added the UN, which has a major peacekeeping operation in South Sudan, was ready to provide "critical support" for a ceasefire monitoring scheme.
The ceasefire was brokered by the Inter-governmental Authority on Development (IGAD), a group of six East African nations, that wants to run the ceasefire monitoring scheme.
IGAD is to have more meetings next week on a follow-up to the ceasefire and how it will be monitored, said African diplomats.
"The UN will continue to protect civilians at risk and calls on all parties to ensure the safety and security of UN personnel and facilities," Haq said.
The UN says both sides have committed "atrocities" in the conflict that erupted on December 15 and is believed to have left several thousand dead.
Haq said there are now 35 000 civilians sheltering at two UN compounds in the capital Juba and 10 300 at the UN compound in Bor, capital of Jonglei state, which has changed hands several times during the battles.
There are now more than 76 000 civilians at eight bases across South Sudan, according to the spokesperson. – Sapa-AFP