South Sudanese government and opposition troops fought 'sporadic' battles after a ceasefire came into force today, the United Nations said.
A spokesperson said the number of people who have sought refuge from the five-week-old conflict in UN bases has now risen above 76,000.
A ceasefire between followers of President Salva Kiir and his former vice president Riek Machar took effect at 1730 GMT.
"The 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.
South Sudanese rebels accused Kiir's army of attacking their positions ahead of the ceasefire, which was brokered by East African nations and agreed yesterday in Addis Ababa. But 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 spokesman said.
Haq added that the United Nations, which has a major peacekeeping operation in South Sudan, was ready to provide "critical support" for a ceasefire monitoring scheme.
"The United Nations 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 United Nations 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.
