JUBA - At least 57 people were killed in tribal clashes on Wednesday in South Sudan's vast Jonglei state, where more than 60,000 people have already fled ethnic violence, the government said on Friday.

South Sudan declared independence from north Sudan in July under the terms of a 2005 peace agreement that ended decades of civil war.

Its government is now struggling to assert control over an underdeveloped territory roughly the size of France.

The country has been plagued by ethnic clashes over cattle, territory and blood feuds for decades but violence has risen, fuelled by a flood of weapons left over from the civil war and other conflicts.  

Fighting between Lou Nuer and Murle groups broke out around two weeks ago in remote regions of Jonglei state, home to a largely-unexplored oil field controlled by France's Total.

At least 57 people were killed in a fresh outbreak of fighting on Wednesday when armed Murle fighters attacked three Lou Nuer villages in Uror country in northern Jonglei state, said government spokesman Barnaba Marial Benjamin.

"The government cannot control everything. It's a huge area ... Eleven men were killed, the rest were women and children," he said.

Benjamin said the attackers had also wounded 53 people and stolen cattle. The government was sending army and police reinforcements to the area in Jonglei, which borders Ethiopia, he added.

The violence started when around 6,000 Lou Nuer men attacked the main Murle town of Pibor and other settlements in southern Jonglei, according to government sources and the United Nations.

Around 20,000 displaced people had started to return to Pibor, but others were still hiding in the bush, said Lise Grande, UN humanitarian coordinator for South Sudan.

"We started yesterday the general deliveries of food ... in two of the most heavily affected areas," she said.

South Sudan's government has declared Jonglei a disaster area.  

Some analysts say South Sudan may become a failed state as the government struggles to end tribal and rebel violence, widespread corruption and build up state institutions.