Sandino said she had travelled to Geneva to issue her warning to top United Nations human rights officials, including deputy UN rights chief Kate Gilmour, whom she is due to meet on Friday.

"I wanted to stress that the progress made towards peace in Colombia with respect to stopping the fighting and the bloodshed... has stopped," Sandino said in an interview.

"We are in a very complex and very difficult phase" for the implementation of 2016 deal to end a half-century of conflict, added Sandino, who helped negotiate the pact.

The ex-rebel Revolutionary Armed Forces of Colombia, now a political party, has criticised the Bogota government over a lack of promised resources to help reintegrate former guerillas into civilian life.

A complicating factor for the deal was the election earlier this year of rightwing President Ivan Duque, who has vowed to fix "flaws" in the agreement negotiated by his predecessor, Nobel Peace Prize winner Juan Manuel Santos.

Duque has in particular criticised the FARC deal as being too lenient in allowing former rebels accused of atrocities to serve as lawmakers.

"Our concern is huge," said Sandino, 53, commenting on Duque's approach to the peace deal.

"There is a lot of scepticism and despair, even among those who lived through the conflict," she added.

Part of her mission to Geneva involves meeting with diplomats from countries that backed the peace process to ensure they help see the implementation through.

The deal has led to a decline in violence but unrest has persisted, notably in the border areas particularly infested by cocaine traffickers.

Colombia remains the world's largest producer of cocaine, much of it destined for the United States.

Some dissident groups also refused to participate in the four-year peace process hosted by Cuba and have continued to fight government forces.

Sandino claimed that 76 former FARC guerillas have been murdered since the deal was signed, while another six have disappeared and 20 relatives of former rebels have been killed.

She did not assign blame for those deaths but warned that the overall human rights situation in Colombia was deteriorating, with activists and civil society leaders facing increased violence.