The United Nations launched an emergency humanitarian effort Friday following last month's intertribal clashes in South Sudan, responding to a wave of violence that might have left thousands dead and some 50,000 people in urgent need of aid.

South Sudan became independent last July following a 2005 peace deal with now-northern neighbour Sudan, and there have been sporadic cross-border attacks since. But internal violence between the Lou Nuer and the Murle tribes is a reminder of the challenges the world's newest country faces inside its own borders.

Last month's clashes took place in and around the town of Pibor, sending tens of thousands of residents into the countryside.

Media reports have put the death toll in the clashes as high as 3,000, but Martin Nesirky, spokesman for U.N. Secretary-General Ban Ki-moon, said the U.N. could not confirm that number. On Tuesday, Lise Grande, the top U.N. official in the region, said the death toll could be anywhere from dozens to hundreds.

Nesirky said a rapid response plan is now being finalized.

"The requirements already reported are already significant and around 50,000 people are estimated to be in need," Nesirky said. The U.N.'s High Commissioner for Refugees, Antonio Guterres, will travelling to South Sudan this weekend to assess the situation, he added.

Officials said the World Food Program has delivered emergency rations to feed 1,000 people in Pibor for two weeks, and expects to reach 7,000 more people in the coming days. It has also distributed food packages for 2,000 internally displaced people at Boma.

On Friday the White House declared the government of South Sudan eligible to receive weapons and defence assistance. A White House official said the decision could potentially promote peace and regional stability in East Africa.

Nesirky said the U.N. mission has reinforced peacekeepers' presence in key areas and is conducting daily land and air patrols to deter potential violence. He said they were also working with the government of South Sudan to protect civilians.

On Thursday, Herve Ladsous, the undersecretary-general for peacekeeping operations, called the situation "a very serious crisis."

"I think the problem we face in this particular region of Jonglei state is one of access, because there are no roads and we have insufficient helicopters," he told reporters following his address to the Security Council. He said the U.N. reinforced its staff in the area and that the South Sudanese government is trying to do the same.

The clashes occurred when columns of fighters from the Lou Nuer ethnic group marched into Pibor to target the Murle community. The tribes have traded violent attacks over the last several years that have killed thousands. Much of the communities' animosity stems from cattle raiding.