Two police officers and a civilian were killed and part of the facility was damaged by a fire during the overnight attack on the central prison in the city of Butembo, said Antony Mwalushayi, a spokesperson for the army's operations against armed groups in Democratic Republic of Congo's North Kivu province.
"The enemy was heavily armed with a headcount of at least 80 individuals. They managed to break (into) ... the prison and free all the prisoners," Mwalushayi told reporters, without saying how many prisoners had escaped.
Prison director Brunelle N'kasa later told Reuters only 58 of 874 inmates remained.
Mwalushayi said he was confident the Allied Democratic Forces (ADF) was behind the attack. The Ugandan armed group, which has been active in eastern Congo since the 1990s, has links to Islamic State and is responsible for repeated massacres.
Butembo Mayor Mowa Baeki-Telly appealed to residents to help round up the prisoners.
"If there is an escapee, he should not be burned - do not kill him ... bring him here to us so that we can put him back in prison," he told reporters.
Escapes are common from Congo's overcrowded and poorly secured prisons. In 2020, the ADF was linked to a prison break that freed more than 1,300 inmates in the eastern city of Beni.
(Writing by Alessandra Prentice; Editing by Mike Harrison)
By Erikas Mwisi Kambale and Fiston Mahamba