Since war broke out in 2013, up to 300,000 people are estimated to have been killed and more than 3.5 million people have been displaced.
The two countries are fueling the war by serving as conduits for arms to combatants, Adama Dieng, the UN special advisor for prevention of genocide told VOA.
“The responsibility to prevent atrocities is regional and international," “It is true that large quantities of weapons and ammunition are flowing into South Sudan through Kenya and Uganda.”
Mr Dieng accusations comes in the wake of another damning report by the UN panel which said in addition to allowing arms to reach government forces, Uganda serves as the destination for teak and gold extracted from South Sudan, which are sold to finance military operations and enrich South Sudanese elites.
South Sudanese rebels and witnesses have also accused Uganda in the past of aiding the government of Salva Kirr by allowing South Sudanese soldiers to pass through its territory to launch assaults against rebels, raising the risk of the civil war spilling over into neighboring East African countries.
Kenya's banking and real-estate sectors, “are key destinations for financial assets and laundered funds from South Sudan,” according to the UN panel report.
Mr. Dieng said peace can only be achieved in South Sudan if the international community and African countries give no alternative to South Sudan leaders but to sit down and talk.
"if we have concerted regional and international efforts to leave no further options to the South Sudanese leaders to stop and start negotiating."
"International partners have to start targeting the accomplices, intermediaries of the South Sudanese parties,” Mr Dieng said.
“Welcoming refugees who are victims of a conflict they are de facto facilitating is not good enough,” he added.
Since war broke out in 2013, up to 300,000 people are estimated to have been killed, more than 3.5 million people have been displaced in a country of about 12 million, with over 2.1 million of those internally displaced, and over 1.5 million having fled to neighboring countries, especially Kenya, Sudan, and Uganda.