Unidentified gunmen killed 23 people when they ambushed a lorry carrying soldiers and civilians in the oil-producing region of south Sudan, an army spokesman said on Wednesday.
The attack came less than a week after 21 others were killed in a cattle raid in the south, as concerns grow that continued violence could unsettle the semi-autonomous region ahead of a January vote on independence.
The south's army spokesman, Malaak Ayuen Ajok, said 17 soldiers and six civilians were killed in the attack, which took place on Sunday but had not yet been reported. A truck they were riding in overturned after the driver was shot and killed.
Ajok said it was not immediately clear who was responsible. "Bandits are active around that area, but lately it has been peaceful. This attack comes as a big surprise."
The ambush took place in Unity State, an oil-producing region whose fields are operated by the Greater Nile Petroleum Operating Company (GNPOC), a consortium led by China's CNPC.
The distribution of oil resources is seen as a key sticking point between the north and south as they negotiate post-referendum arrangements. Ajok dismissed any connection between the oil dispute and the latest attacks in Unity State.
Most observers believe southerners will vote for secession on Jan. 9 after fighting the north on and off since 1955 over issues including oil, religion, ethnicity and ideology.
The south also has a history of cattle raids and tribal clashes, and some fear bloodshed around the vote. Violence has intensified as weapons left over from Sudan's civil war have flooded the region since the north-south peace deal in 2005.
Separately, Ajok said the south's army had captured a helicopter carrying rebel soldiers and munitions on it's way back to Khartoum after supplying a renegade southern army general, George Athor.
Athor turned rogue after losing in disputed April elections and has been in hiding, emerging only for sporadic clashes with the army, ever since. South Sudan's government has said it suspected their former northern foes were supporting his revolt, and Ajok said the captured helicopter was proof.
"No one could get such a helicopter or get those munitions without a green light from the Sudanese Armed Forces," Ajok said, referring to the army of the north.