The ombudsman's office posted on social media that about 60 people have died violently, 32 were kidnapped and hundreds were displaced due to conflict in recent days between the leftist National Liberation Army (ELN) and the now-demobilized Revolutionary Armed Forces of Colombia (FARC), Reuters reported.
President Gustavo Petro last week accused the ELN of committing a war crime in the attacks, and suspended peace talks with the group.
The ELN, in a statement on Saturday, accused a faction of the FARC of having initiated the conflict by killing civilians and committing other aggressions.
The FARC faction did not publicly respond to the allegation. On Friday it said it had begun withdrawing its units to prevent the violence from escalating.
The Catatumbo region is considered strategic for drug trafficking because of its proximity to Venezuela, from where illegal armed groups export cocaine, security sources have said.
Petro's government since 2022 has pursued peace negotiations with leftist guerrillas and criminal gangs founded by former right-wing paramilitaries, in a bid to end Colombia's internal conflict.
Six decades of fighting in the Andean country has killed at least 450,000 people.
The ombudsman's office warned that many social leaders and their families are at risk of being kidnapped or killed for opposing the ELN. It called on the ELN and other armed groups to allow humanitarian aid into the area and to "end all attacks against the civilian population."