Friday, "Khartoum-supported mercenaries" attacked an SPLA position in a town called Wau, causing 21 deaths, he said.
The Khartoum government and the SPLA have recently called a ceasefire and are discussing a possible settlement.
SPLA's fighting ability, it has failed to capitalise on the rebel army's disorganisation.
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Sudan is fighting a bitter conflict against the SPLA-North, a rebel group that wants to be part of independent South Sudan.
At least four SPLA soldiers were hurt in the attack, she said.
"I fought with the SPLA, and now we have won, " said Gony Thon, a policeman, joining in the impromptu dance on the street.
In 2011 he accepted an amnesty offer by President Salva Kiir and returned to Juba where he was promoted to an SPLA general.
SPLA. At the same time the contest over Sudan's scarce resources has evolved into a fight for control of the south's abundant oil reserves.
But if the military tide were to turn against the SPLA-N there would be fierce popular pressure on the southern government to help it wholeheartedly.
The SPLA and the government have for several years agreed to temporary ceasefires in limited areas of the conflict zone, to allow the passage of food aid.
An attack on the town's southern flank had been repulsed after the SPLA, backed by UN armoured personnel carriers, had fired at the Lou Nuer, she said.
He said the SPLA had contacted the U.N. mission after seeing the helicopter near an SPLA command post but had been told it had no aircraft in the area.
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SPLA's long-awaited (and Ugandan-backed) September offensive in southern Sudan.
Aguer said the southern military, known as the SPLA, would continue to "deal with the militia group" and that it would be a matter of time before the rebels are cleared from Pibor County in the state of Jonglei.
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