Ugandan forces worked in concert with South Sudan’s military to launch airstrikes that killed and severely burned civilians, a United Nations investigation has found, raising fresh questions about regional involvement in South Sudan’s escalating conflict.

The U.N. Commission on Human Rights in South Sudan said joint aerial operations by the two countries targeted civilian-populated areas, hitting communities largely inhabited by Nuer people in territories associated with the opposition. The commission described the strikes as part of a broader campaign to weaken forces loyal to opposition leader Riek Machar.

Ugandan troops have been deployed in South Sudan under a bilateral security agreement, officially to support President Salva Kiir’s government against armed groups. Kampala insists its forces are present only in an advisory and protective role and denies taking part in combat operations or attacking civilians.

The U.N. report, however, details what it calls “high degrees of planning, operational integration and command-level authorization” between the two militaries. Investigators say the attacks relied heavily on improvised incendiary devices, including barrels filled with flammable liquid dropped from aircraft over populated areas.

One of the most devastating incidents documented occurred near Wunaliet, about 15 kilometers from Juba. Witnesses told U.N. investigators that planes released barrels of liquid that ignited on impact, engulfing homes in flames. Survivors described seeing civilians set alight, including a boy burned beyond recognition. A nearby barracks housing opposition fighters was also hit.

Shortly after that attack, Gen. Muhoozi Kainerugaba, Uganda’s top military commander and son of President Yoweri Museveni, posted on X that Ugandan forces had bombed opposition positions in South Sudan. “Our air offensive will not stop until Riek Machar makes peace with my uncle Afande Salva,” he wrote, in a message later deleted. The post was accompanied by video footage appearing to show fiery explosions filmed from an aircraft.

Flight-tracking data reviewed by U.N. investigators indicated that a Ugandan military turboprop flew from Uganda into South Sudan and circled the Wunaliet area during the bombardment.

Uganda has rejected allegations that it used chemical weapons or barrel bombs and maintains it complies with international law. Rights groups, including Amnesty International, say Uganda has already breached a U.N. arms embargo on South Sudan by providing military support, deepening fears that the fragile 2018 peace deal is at risk of collapse.