|South Sudan has seen an increase in violence since its vote to secede from the North in January [GALLO/GETTY
Heavy fighting between rebels and the south Sudanese army in the oil-producing states of Unity and Upper Nile, has left at least 70 people dead, according to an army spokesman.
At least 30 soldiers and 11 rebels died in clashes that broke out on Thursday morning in Mayom county, in Unity state, Philip Aguer, a spokesman for the south's Sudan People's Liberation Army (SPLA), said.
"The fighting was very heavy, but the rebels are now being pursued," Aguer said, who added that 34 SPLA soldiers were also wounded.
"The group has split into two groups - one going towards the border with the north and another into a swamp area."
Aguer said the fighting began after the breakdown of talks aimed at persuading the rebels, who totalled "several hundred well-armed men," to join the southern army.
In neighbouring Upper Nile state, clashes between the army and a separate rebel group, which Aguer said was responsible for a bloody raid on the state capital Malakal last Saturday, killed 25 rebels and four SPLA soldiers.
"The rebel dead are 25 and there are two captured vehicles, while on the side of the SPLA four were killed," the army spokesman said.
The rebels' commander, known as Ulony, was wounded in both arms, but was believed to have escaped across the border into north Sudan.
"Our troops chased him and the rest of his forces but they could not continue across the border," Aguer said.
UN officials confirmed that heavy clashes had taken place, but they were unable to verify the casualty figures.
South Sudan has seen a surge in bloody clashes between rebel groups and the army since a largely peaceful January referendum on independence, in which southerners voted almost unanimously to form their own nation.
Unity and Upper Nile states are close to the south's border with the north - its former civil war enemy who the southern government has repeatedly accused of supplying arms and providing bases for rebels.
Talks to resume
The leaders of north and south tentatively agreed on Thursday to resume talks aimed at resolving a host of outstanding issues before the international community recognises an independent south in July.
The talks had broken off last week amid bloody clashes in the south and accusations of a northern conspiracy.
Pagan Amum, the secretary-general of the south's ruling Sudan People's Liberation Movement (SPLM), has charged that the Khartoum government is arming Arab tribes all along the north-south border in a policy of attempted genocide.
Southern officials have also released documents they say details northern arms shipments to southern militias.
The north has repeatedly rejected such accusations, insisting the documents were forgeries.
Analysts have warned that security will pose a major challenge for the southern authorities in the run-up to recognition.