The Conflict Armament Research (CAR) lobby group has accused Uganda of illicit supply of arms to South Sudan.
The London-based CAR says in its latest report that Uganda was buying weapons from European countries and giving them to South Sudan, despite an arms embargo.
It accuses Kampala of violating the European Union arms embargo on South Sudan by channelling to the war-ravaged state weapons purchased from Bulgaria, Slovakia and Romania.
Juba, the CAR findings say, uses Kampala to act as a key arms dealer, especially after the imposition of an embargo by the UN Security Council in July.
“South Sudan arranged for Uganda to issue end-user certificates (the essential paperwork for an international arms transfer)…to make it look like these weapons were for the use of the Ugandan armed forces when in fact they were always destined for South Sudan,” Mr Mike Lewis, the head of CAR regional operations was quoted in the report as saying.
CAR also says that some of the ammunitions transferred to South Sudan by Uganda were discovered in the hands of Sudanese rebel groups fighting to dislodge President Omar Bashir.
Attempts to get Juba's response were futile.
Recently, the UN Panel of Expert on South Sudan also claimed that Kampala and Khartoum were both violating the arms embargo imposed on Juba.
It said Kampala was openly giving military support to Juba.
Uganda's President Yoweri Museveni has since the beginning of the civil war in South Sudan, provided direct support to President Salva Kiir in the war with his former deputy Riek Machar.
The two foes signed a peace deal recently, brokered by Uganda and Sudan, but which faces scepticism from the West, especially the US and the UK who view Kampala and Khartoum as double dealers.