South Sudan rebels and government forces have accused each other of violating the juvenile nation's latest ceasefire deal.
Massive international pressure led to them agreeing to allow humanitarian access two days ago.
Rebels in South Sudan accused government forces of multiple ceasefire violations on Sunday -- just hours after a fresh ceasefire came into effect late Saturday. A similar bid fell apart in January.
Rebel military spokesman Lul Ruai Koang was quoted by the news agency AFP as claiming that government army units had staged ground attacks and artillery barrages in the oil-rich northern states of Upper Nile and Unity.
Army spokesman Philip Aguer said government forces were attacked in Unity State, with one incident near the flashpoint oil hub of Bentiu. The town has changed hands several times in recent weeks.
There was no immediate independent verification of either set of claims, nor confirmation from the UN mission in the region.
Earlier, in the capital Juba, the government of President Salva Kiir said his forces had been given strict orders to respect the peace deal.
On Friday, Kiir and his rival Riek Machar signed the deal in Ethiopia's capital Addis Ababa to halt fighting within 24 hours.
Massive international pressure to do so followed UN warnings that South Sudan was heading for a major humanitarian crisis and even genocide.
The deal included a pledge to freeze troop movements and allow access for humanitarian aid organizations. That was to be followed by more deescalation talks aimed at forming a transitional government of national unity.
Since December, warfare, largely along ethnic lines, had killed thousands and displaced more than a million people, including many children (pictured) sheltering in refugee camps.
Famine is feared if displaced farmers cannot access their fields before seasonal rains expected by the end of May.
Impact will linger
The regional South Sudan director for the aid organization Oxfam, Cecelia Millan, said "even if peace holds" the impact of the crisis would "linger into next year."
"A mammoth aid effort needs to be mobilized to meet the enormous human need," Millan added, because fighting had also disrupted food and medical deliveries.
On Saturday, EU foreign policy chief Catherine Ashton urged Kiir and Machar to continue talks.
The latest ceasefire bid was brokered by top UN and US officials as well as African regional blocs.