Fighting has been reported between government and rebel forces in South Sudan after a ceasefire came into effect, the United Nations said.
"The UN Mission in South Sudan says that sporadic fighting took place in parts of the country today," UN deputy spokesman Farhan Haq told reporters on Friday. He added that some of the battles came after the ceasefire came into effect at 17:30 GMT.
Rebels accused President Salva Kiir's army of attacking their positions ahead of the ceasefire, which was brokered by East African nations and agreed on Thursday 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.
He 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.
For the people of South Sudan, I am pleased to tell them that the conflict that erupted in December, that was uncalled for, will be resolved through peaceful dialogue.
The reports of continuing conflict reflect the deep distrust between Kiir and rebel fighters, who include those loyal to former Vice President Riek Machar, whom Kiir fired, as well as more autonomous groups.
Kiir said the deal signed on Thursday would bring an end to suffering for South Sudan's people, a half million of whom have fled their homes since violence began on December 15. Weeks of fighting has killed thousands.
"For the people of South Sudan, I am pleased to tell them that the conflict that erupted in December, that was uncalled for, will be resolved through peaceful dialogue,'' he told a news conference.
But as a sign of the government's own concern about the strength of the agreement, Kiir also appealed to the rebels to accept the deal, with a message directed at armed youths known as the White Army, who are loyal to Machar.
In Ethiopia, where Thursday's deal was signed, Brigadier General Lul Ruai Koang, a rebel spokesman, said government forces were attacking rebel positions in oil-rich Unity state and in Jonglei state.
Koang called the attacks a "clear violation" of the peace deal and said rebel forces would defend themselves against attacks.
But Kiir's spokesman dismissed the claim SPLA government soldiers had launched any strikes.
"There was no single gunshot anywhere and we hope the rebels will honour their signature," spokesman Ateny Wek Ateny told a news conference in the South Sudanese capital Juba.
The World Food Programme said on Friday looters had stolen more than 3,700 tonnes of food, enough to feed 220,000 people for a month. The UN agency said its warehouses in the northern town of Malakal had been almost emptied.
US President Barack Obama said on Thursday the ceasefire was a critical first step towards peace in the world's newest state. But he added leaders needed to work to resolve the underlying causes of the conflict.
Diplomats say the deep political, personal and ethnic grievances will be hard to overcome.