Kenya and Uganda are aiding to prolong the four-year-old civil war in South Sudan by serving as conduits for arms to combatants, a United Nations official said on Monday.
"The responsibility to prevent atrocities is regional and international," Adama Dieng, the UN special advisor for prevention of genocide, told VOA.
"It is true that large quantities of weapons and ammunition are flowing into South Sudan through Kenya and Uganda."
Mr Dieng said peace will be achieved in South Sudan only "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.
Uganda is hosting more than one million refugees from South Sudan, while Kenya's Kakuma camp holds more than 100,000.
Mr Dieng did not indicate whether the governments of Kenya and Uganda are directly involved in arms trafficking to South Sudan. He also did not say whether the weapons are intended for the country's military or rebel forces -- or possibly both.
The UN panel of experts reported last November it had obtained documentary evidence of a cargo flight containing 31 tonnes of weapons that arrived in Entebbe, Uganda, in August.
Kampala-based Bosasy Logistics was listed as consignee for the shipment which was said to have originated in Bulgaria. The arms were to be transferred to South Sudan, according to unnamed sources cited by the UN experts.
Mr Dieng's contention that Kenya and Uganda are fuelling the war in South Sudan follows a comment by US Ambassador to the UN Nikki Haley last week that "it is past time for the leaders of Uganda and Kenya to get involved and put pressure on President Kiir".
Kenya and Uganda "are key players in the success of a true peace process," Ms Haley said in a speech to the UN Security Council.
UN Secretary-General Antonio Guterres also recently warned South Sudan's neighbours against taking sides in the civil war.
While not naming Kenya or Uganda, the UN chief told an African Union gathering in Addis Ababa on January 27 that it is essential to ensure that "any contradictions that might exist among the neighbours of South Sudan are not translated into an influence in the internal situation of South Sudan."