JUBA, South Sudan -- Fresh violence erupted between two rival tribes in South Sudan's troubled Jonglei state, officials said Thursday, giving no details of casualties in the region that is also the scene of an ongoing military assault against a rebel group.

South Sudan army spokesman Col. Philip Aguer said ethnic clashes had erupted between the rival Lou Nuer and Murle tribes in Pibor county.

Jonglei's mayor Kuol Manyang confirmed fighting was ongoing but provided no details.

Aguer said South Sudan's army had not received orders to intervene in the ethnic clashes that he said were a "communal" issue to be handled by the civilian authorities.

"If there are two communities fighting how do you separate them with firearms?" Aguer asked. "Since these are civilians fighting civilians we think it is the (civilian) authority that should come up with a decision."

This position was criticized by the U.S. Embassy in Juba, which in a statement Wednesday condemned the violence "in the strongest possible terms" and urged the government and community leaders to "call on all armed youth to lay down their weapons immediately."

"We are deeply disappointed the SPLA did not establish a posture appropriate to defend civilians in vulnerable areas, despite advance warning of the mobilization of armed youth which has led to the current violence," the statement said. "The lack of action to protect civilians constitutes an egregious abdication of responsibility by the SPLA and the civilian government."

South Sudan, which marked its second independence anniversary Tuesday, has been plagued by a border conflict with Sudan and ethnic violence that has killed many and forced thousands to flee their country. This is the second bout of violence since suspected Murle youths attacked members of the Lou Nuer in May.

South Sudan's army is also in Pibor county battling a rebel militia led by a renegade colonel named David Yau Yau.

Aid workers and rights groups have accused both the South Sudanese military and Yau Yau's rebel group of abusing civilians and denying them access to humanitarian assistance.

The latest violence comes just as South Sudan celebrates its second anniversary of independence from Sudan. Over the two years, the two Sudans have come close to fighting a war over oil and border disputes, as well as accusations of supporting rebels across the border.

On Monday a group of Americans who described themselves as "friends" of South Sudan sent an open letter to South Sudan's president in which they urged him to do more to prevent the country from becoming another failed state.

"Some of us have communicated our concerns with you individually and confidentially in the past, always as friends," the letter said. "At this moment, our friendship dictates that we express our concerns about the increasingly perilous fate of South Sudan. From our various vantages, we have all come to conclude that without significant changes and reform, your country may slide toward instability, conflict and a protracted governance crisis."