A statement from the rebels accused government soldiers loyal to President Salva Kiir of launching ground attacks and artillery barrages against their positions in two oil-rich northern states, while the government insisted the rebels attacked first.
"The violations of the Agreement to Resolve the Crisis in South Sudan shows that Kiir is either insincere or not in control of his forces," rebel military spokesman Lul Ruai Koang said.
The two sides had agreed to a ceasefire in January, but that deal quickly fell apart and unleashed a new round of fierce fighting.
Clashes were also reported around the northern oil hub of Bentiu -- the Unity state capital which has changed hands several times in recent weeks and has been described as being particularly tense.
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Independent aid sources in the town confirmed they could hear fighting, but added it was impossible to say which side started fighting first.
But South Sudan's defence minister, Kuol Manyang, told AFP that it was the rebels who attacked first in Bentiu and that the opposition suffered heavy casualties.
"They attacked first thing this morning. They attacked our position and 27 of them were killed. They have a policy of attacking then going to the media," he said.
Kiir's spokesman, Ateny Wek, also asserted that government commanders were under clear orders not to fight, unless in self-defence.
"The orders have been given to the army to start observing arrangements for the cessation of hostilities," he said, adding: "The right to self defence had not been waived by the agreement.