South Sudan's rebels have accused government forces of multiple ceasefire violations along several fronts, just hours after a truce aimed at ending the five-month conflict came into effect.

Rebel military spokesman Lul Ruai Koang alleged in a statement that the violations occurred in the oil-rich northern states of Upper Nile and Unity.

He said they included both ground attacks and artillery barrages. He added that rebels reserved "the right to fight in self-defence".

Clashes were also reported around the northern oil hub of Bentiu - which has changed hands several times in recent weeks and has been described as being particularly tense.

In the capital Juba, however, the government said their forces had been given strict orders to its troops to respect the peace deal.

President Kiir's spokesman Ateny Wek said the government had received no word from the rebels, but added that government army commanders "have sent any reports of violations" by forces loyal to rebel leader and former vice-president Riek Machar.

Mr Kiir and Mr Machar met in the Ethiopian capital Addis Ababa on Friday and agreed halt fighting within 24 hours - or by Saturday evening.

The agreement came after massive international pressure on both sides to stop a five-month conflict marked by widespread human rights abuses, a major humanitarian crisis and fears the world's youngest nation was on the brink of a genocide and Africa's worst famine since the 1980's.

The war has claimed thousands, and possibly tens of thousands, of lives, with more than 1.2 million people forced to flee their homes.

Mr Kiir and Mr Machar had agreed to a ceasefire in January, but that deal quickly fell apart.