The International Committee of the Red Cross (ICRC) said it documented countless atrocities last year even as the government closed in on a key peace accord with the FARC rebel force.
It "documented 838 possible violations of international humanitarian law and other humanitarian principles that have affected more than 18,600 people," the body said in an annual report.
It said 40 percent of the atrocities targeted women and children.
The deal signed in December with the leftist Revolutionary Armed Forces of Colombia (FARC) "will not be enough to end the violence in Colombia," it warned.
"Building peace requires an effort by everyone and will take decades."
The FARC is starting to disarm under the December deal. The government has also launched formal talks with the last remaining Colombian rebel force, the leftist National Liberation Army (ELN).
A major world aid body, the ICRC has been present in Colombia for 40 years and has acted as a neutral mediator in hostage negotiations between the state and rebel groups.
It echoed recent warnings by officials and civil groups that despite the peace efforts, remnants of right-wing paramilitary groups remain active in the conflict zones, as well as organized gangs.
The ongoing violence is continuing to displace civilians who get caught in the crossfire, said the ICRC chief in Colombia, Christoph Harnisch.
He said nearly 86,000 people are currently missing due to the conflict.
Colombian authorities say the conflict has killed 260,000 people and displaced 6.9 million since it started with a leftist uprising in 1964.
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