Uganda fears a full-blown conflagration in its oil-producing northern neighbour could send waves of refugees across the border and potentially create instability.
“The Ugandan forces are taking part in air strikes against civilians,” Machar said in the March 23 letter, urging pressure on Uganda to withdraw its troops.
A spokesperson for Machar's office verified the authenticity of the letter seen by Reuters.
Uganda and South Sudan's military spokespersons did not immediately respond to requests for comment about the potential violation of the embargo, which has been in place since July 2018.
South Sudan's army attacked SPLM-IO forces stationed at a camp near the capital Juba on Monday night, the party's military spokesperson Lam Paul Gabriel said on X. South Sudan's information minister did not immediately respond to the accusation.
Last week Uganda's parliament retrospectively approved the deployment in South Sudan, first announced on March 11.
Ugandan defence minister Jacob Markson Oboth said the deployment was “to avoid a security catastrophe” in Africa's newest nation.
In a series of since-deleted posts published on X in the early hours of Sunday, Uganda's military chief Muhoozi Kainerugaba said: “I'm tired of killing Nuer,” referring to Machar's ethnic group.
“Tell your leader Riek Machar to come and kneel down before 'our' President Salva Kiir,” wrote Kainerugaba, who has a history of making inflammatory statements that have previously sparked diplomatic tensions in the region.
**Reuters**