An international monitoring group says it has evidence that Sudan has been air-dropping arms and ammunition for use by South Sudan rebel forces.

The London-based Conflict Armament Research (CAR) group reported last week that their investigators had found evidence of the Sudan supply operation during a field visit to a South Sudan People’s Army (SPLA) base in Malakal in Upper Nile State last December.

Among the seized materiel shown to the researchers were Chinese assault rifles, rocket launchers, machine guns and recoilless rifles that had their serial numbers removed by Sudan state agents, the research group says in its report.

“The deliberate removal of serial numbers is a common feature of weapons supplied by Sudan to South Sudanese rebel forces,” the group says in a report.

The weapons and ammunition seen by the investigators showed signs of having sustained the type of damage that results from impact from a high-altitude drop.

The report could vindicate the South Sudan government, which has claimed several times that Khartoum is supporting the rebels, following Sudan’s claims that Juba has been supporting the SPLM-North in Southern Kordofan and Blue Nile, and the Justice and Equality Movement (JEM) in Darfur.

In late May, South Sudanese military spokesman Philip Aguer alleged that Khartoum had a hand in the fresh fighting in Malakal, the capital of Upper Nile State, and in Unity State where the rebels threatened to capture the oil fields.

Col Aguer made claims similar to those contained in the report that the weapons captured from rebels show they were supplied by Khartoum.

But Sudanese Foreign Ministry spokesman Ali al-Sadiq, in an interview with the Sudanese News Agency, maintained that his country has “no interest in such an intervention,”  and that the instability in South Sudan would send more refugees into Sudan and create instability hotspots along its border.

In a March interview with The EastAfrican, rebel leader Riek Machar denied that Khartoum was supplying him with weapons, claiming that the weapons his group was using were captured from the government, which is directly supplied by China.

Jonah Leff, the research group’s director of field operations, in an interview with The EastAfrican, maintained that SPLA does not have in its own stockpiles the types of ammunition shown to the investigators.

“Some of this Chinese war-fighting equipment is transferred to South Sudan rebel forces in violation of end-user agreements between Beijing and Khartoum. It also contravenes several international and regional agreements barring the provision of arms to non-state groups,” Mr Leff said.

Although CAR could not independently verify the sequencing and number of reported airdrops, the report says that observers on the ground reported that throughout the month of December 2014, aircraft dropped additional military equipment to Sudan People’s Liberation Movement-in-Opposition (SPLA-IO) units in Upper Nile and Jonglei States.

The marks on the rifle’s receiver had been deliberately removed by grinding prior to its capture by the SPLA. However, secondary marks on the weapon’s recoil spring guide remained intact.

“The types of materiel air-dropped to support operations in Jonglei State mirror weapon types that Sudan has supplied to support armed or insurgent forces elsewhere in sub-Saharan Africa, including in Darfur and the Central African Republic,” the researchers’ report says.

Mr Leff said that although China had stopped direct supply of arms to the South Sudan government, the SPLA makes use of Chinese military supplies shipped at earlier stages of the 18-month-old conflict.

Additional reporting by Fred Oluoch