UN secretary-general Antonio Guterres condemned the attack in a statement, urging “the importance of collective efforts, including through enhanced regional partnerships, to tackle cross-border insecurity between (DR Congo) and Uganda and restore durable peace in the area”.
The atmosphere in Mpondwe-Lhubiriha was tense but calm on Sunday, as Ugandan security forces roamed the streets outside and near the school, which was protected by a police cordon.
Ugandan security forces have not given a detailed account of how the suspected rebels were able to carry out the attack.
Please use Chrome browser for a more accessible video player
1:20
Some students have been abducted while others suffered fatal burns, were shot, or hacked with machetes.
In a statement released on Sunday, Ugandan president Yoweri Museveni described the attack as “criminal, desperate, terrorist and futile”, vowing to deploy more troops on the Ugandan side of the border.
The ADF has been accused of launching many attacks targeting civilians in remote parts of eastern DR Congo in recent years, including one in March in which 19 people were killed.
The group has long opposed the rule of Ugandan president Mr Museveni, a US security ally who has held power in the East African country since 1986.
The ADF was established in the early 1990s by some Ugandan Muslims, who said they had been sidelined by Mr Museveni’s policies.