The Allied Democratic Forces (ADF), a rebel group, are alleged to have killed more than 40 people in an attack on Mayikengo village this week and over 80 in attacks on other villages in the province the previous week.
The victims were hacked to death with machetes while they were in their homes or the fields, the official said, adding that some were executed while they were trying to escape.
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Goma has a resident population of one million, with another million displaced by the war crammed into desperate, temporary camps that have become the frontlines.
Three other TPDF soldiers are nursing serious injuries from the attack, according to SADC.
The UNHCR is calling for an immediate end to the violence and urges all parties to the conflict to respect and uphold international humanitarian law and human rights, and to protect civilians.
The United States-based Center for Civilians in Conflict on Tuesday denounced the use of heavy artillery and mortar shells against civilian installations.
The incident happened three days after a mortar bomb landed inside one of the South African Contingent military bases.
The fighting between the local army and the M23 has displaced at least 130,000 people in different areas of the Masisi territory in the last two weeks.