Judge Camilo Suarez on Tuesday ordered seven leaders of the Revolutionary Armed Forces of Colombia (FARC) to make reparations for more than 21,000 kidnappings during the group's withering half-century war with the state.

It was the first sentence handed down by the Special Jurisdiction for Peace, known by its Spanish acronym JEP, which was set up under a landmark 2016 peace agreement between FARC and the government.

The defendants -- including the Marxist group's leader Rodrigo Londono Echeverri, alias Timochenko -- were spared prison time and instead told to take part in acts of restorative justice for eight years.

French-Colombian former presidential candidate Ingrid Betancourt, who the group held for more than six years, told AFP she felt "humiliated" because she believed the ruling benefited FARC's leadership.

Judge Suarez told AFP in an interview that it was "absolutely understandable that there are victims who are dissatisfied" because "these are crimes that cannot be repaired."

Some 4,300 kidnapping victims are registered with the court and "listening to each one of them is absolutely difficult, almost impossible," the judge added.

Soldiers, police officers, business owners and political leaders were abducted by FARC over the course of its bloody campaign for a communist state and whisked off to bases deep in the jungle, sometimes never to return.

The seven defendants accepted responsibility for 21,396 kidnappings and were given the maximum sentence provided for under the peace deal.

Suarez said the defendants "must remain in a geographical area determined by the court," which is not meant to "confine" them but to ensure their attendance.

The reparations include activities to promote healing, to help locate missing victims of the conflict and mine clearance campaigns.

"They submitted to the state with absolute commitment" and "total willingness," Suarez said.

The peace tribunal's ruling comes as Colombia suffers its worst violence in a decade, with FARC dissidents who refused the peace deal and other armed groups stepping up their attacks on the state after the collapse of peace talks.

Over 130 soldiers and police were killed in such attacks between January and mid-August, defence ministry figures show.

Betancourt and other victims have said they will challenge JEP's decision before international bodies.

"They are within their rights," Suarez said, adding that such an appeal was "possible" as long as international courts agree to assume jurisdiction.