(adds U.N. reaction, paragraph 5)

By Skye Wheeler

JUBA, Sudan, Aug 3 (Reuters) - More than 160 people were killed when heavily armed South Sudan tribal fighters launched a dawn raid on a rival group, officials said on Monday, the latest in a series of bloody ethnic clashes.

Most of the victims were women and children when men from the Murle ethnic group attacked a camp in the Akobo area of the region's swampy Jonglei state, where oil exploration is under way, on Sunday morning, officials said.

"100 women and children, 50 men and 11 SPLA (soldiers from the southern Sudan People's Liberation Army) are being buried by the riverside this morning," Akobo commissioner Goi Jooyul Yol said in a statement on Monday.

"There may still be bodies in the bush, we don't yet know the full number," Yol later told Reuters by telephone.

U.N. Secretary-General Ban Ki-moon said in a statement issued by his press office that the attack was a "heinous act" and urged the government of southern Sudan "to bring to justice those responsible for these events and take the necessary measures to protect civilians across southern Sudan."

Yol said a small force of southern soldiers that had been stationed in the area to protect the settlement was overrun by the attacking Murle.

Officials said most of the victims were from the Lou Nuer group, locked in a tribal war with the Murle that has already claimed over 700 lives this year in attacks and counter-attacks.

Analysts say the extensive targeting of women and children, and the number of dead, mark a worrying new development in this year's violence.

The south's President Salva Kiir has blamed political agitators who he said want to show that the south cannot run itself ahead of a promised 2011 southern referendum on separation from northern Sudan.

Disputes, many sparked by cattle rustling, have been exacerbated by a ready supply of arms left over from the two-decade civil war between north and south Sudan, and political disaffection over the slow pace of development in the region.

South Sudanese and U.N. officials had hoped the recent onset of the region's rainy season would reduce the violence, as heavy downpours restricted access to remote villages.

"This year there has not been enough rain to lessen the movements," Jonglei deputy governor Hussein May Nyuot said.

Nyuot said the Lou who were attacked were camped beside the Geni River, 40 km (25 miles) southwest of Akobo town. A lack of water this year has driven many communities to set up settlements close to rivers where they can also fish for otherwise scarce food. (Additional reporting by Louis Charbonneau; Editing by Andrew Heavens, Tim Pearce and Bill Trott)

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