At least 20 soldiers were killed and many were injured in an ambush in South Sudan, as the new nation's army hunted rebels in the troubled eastern state of Jonglei, a local official said Friday.

The attack happened Thursday as South Sudan's army headed for rebel leader David Yau Yau's hideout around Akelo, roughly 35 miles (55 kilometres) from Pibor, Pibor Commissioner Joshua Konyi told AFP.

"The Sudan People's Liberation Army (SPLA, the regular army) were trying to cross the river but the rebels wanted to stop the army," Konyi said.

Yau Yau, a former theologian and soldier, mounted a new rebellion in Pibor County last year after large-scale massacres and an army-led disarmament process marred by abuses against civilians.

Konyi said 30 soldiers had been taken to a hospital run by aid agency Doctors Without Borders (Medecins Sans Frontieres, MSF) in Pibor, but added the army offensive would continue.

"The SPLA is moving forward in Akelo, where Yau Yau has his headquarters," Konyi said.

Spokesmen from South Sudan's government and military as well as the United Nations peacekeeping mission, which has troops stationed in Pibor, did not respond to calls from AFP.

On the rebel side "not less than 32 were killed," Konyi claimed.

It is impossible to verify information in the vast and remote area that last January was engulfed by violence.

Less than six months after South Sudan declared independence after decades of civil war with the north, some 8,000 armed youths from the Lou Nuer ethnic group rampaged through Pibor County, vowing to exterminate their cattle-keeping rivals the Murle.

The UN estimated that over 600 people were killed and around 300 in smaller revenge attacks, while Pibor officials said that over 3,000 died in the original attacks.

A subsequent state-wide disarmament campaign led by security forces and mired in claims from rights groups of abuses against civilians, pushed people towards Yau Yau's militia.

After numerous failed or stalled peace talks the army has declared that the amnesty afforded to numerous other rebels in South Sudan has expired for Yau Yau.