The Russian mercenary group Wagner has been supplying the Rapid Support Forces (RSF) paramilitary group in fights against the Sudanese army, according to CNN.
Sudan has fallen into fierce fighting, edging on civil war in recent weeks as two generals vie for control of the country.
CNN cited sources in the country and satellite imagery in Libya, where Wagner-controlled bases show high activity.
Wagner, which is headed by Russian oligarch Yevgeny Prigozhin, has been an influential part of Russian influence worldwide and in the war in Ukraine. The group has been accused of committing some of the most gruesome atrocities of the war.
Wagner mercenaries have a significant presence in Syria, Libya and West Africa. Russian interest in Sudan has peaked in recent years as the Kremlin targets gold to dodge Western sanctions.
Satellite imagery reportedly shows a Russian cargo plane moving back and forth between Libyan air bases in the days leading up to and at the start of the conflict. Russia also reportedly airdropped some mutations to fighters in Sudan.
The fighting in Sudan has killed hundreds, including at least one American. The U.S. is preparing military forces in nearby Djibouti to evacuate the U.S. embassy in Khartoum if necessary.
Libyan warlord Khalifa Haftar, a general who controls vast swaths of the country in its civil war, is backed by Wagner. Sources say that Wagner and Haftar could be preparing to back the RSF in the conflict against the Sudanese army, CNN said.
The RSF has denied receiving help from Russia or Libya.