In a confidential report to the Security Council obtained by AFP on Thursday, the panel described the arms deals that are not recent and involve Israeli and Bulgarian firms.
The council has threatened to impose an arms embargo on South Sudan to try to end the fighting that has killed tens of thousands of people and driven 2.5 million from their homes.
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While the arms deals date back to 2014 or earlier, "this evidence nevertheless illustrates the well-established networks through which weapons procurement is coordinated from suppliers in Eastern Europe and the Middle East and then transferred through middlemen in eastern Africa to South Sudan," said the report.
The panel said rebel fighters loyal to Reik Machar recently turned up in the Democratic Republic of Congo armed with Israeli-made automatic rifles that were part of a stock sold to Uganda in 2007.
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The weapons were likely taken from South Sudanese government stocks either through battlefield capture or defections, said the report sent to the council last week.
The panel said the Israeli-made rifles were likely part of a larger group of weapons that was transferred to South Sudan from Uganda.
After receiving a tip from Spain, the UN experts are looking into an arms trafficking network based in Europe that received an "extensive list of small arms, munitions and light weapons" from the rebels in 2014.
The deal which also involved a middleman from Senegal provided for shipments that were at least partially delivered, they said.