The study from the London-based small arms research organisation, Conflict Armament Research, documented a cache of weapons and ammunition captured by South Sudanese government troops from the rebels in Jonglei State in November.
"The composition of the documented materiel, and a precedent for airdropping identical materiel to rebel forces in South Sudan in 2012, prior to the current conflict, suggests direct supply from Sudan to SPLM-iO (Sudan People's Liberation Movement-in-Opposition) forces," the report said.
The 7.62x39 mm ammunition -- bullets used in Kalashnikov-style assault rifles -- had "lateral impact damage, which is consistent with either free-fall dropping from an aircraft or very low altitude parachute dropping."
Some 70 per cent of this particular type of rifle round were made in Sudan in 2014, the report said.
More From This Section
"The recent date of manufacture is further evidence of direct supply from Khartoum to SPLA-iO forces," the group said.
Other ammunition seen by the team originated in China.
South Sudan split from the north in 2011 under a peace deal that ended a bloody 22-year civil war, but relations between the countries have been tense since then.
Khartoum has repeatedly denied that it is involved in the civil war in South Sudan, which has pitted President Salva Kiir against his arch rival and former deputy Riek Machar since December 2013.
Fighting in South Sudan has worsened in recent weeks, with the UN and aid agencies saying millions of people are going hungry.
Several rounds of peace talks between the rebels and the government have done little to put an end to the conflict.