The senior presidential adviser for Colombia's post-conflict phase confirmed Thursday that a United Nations official had been kidnapped a day earlier by leftist rebels but said he would be released immediately.

"The UN official is to be released at midday. He was kidnapped by a dissident faction of the FARC (Revolutionary Armed Forces of Colombia), and they've already announced that they would free him. This action is absolutely unacceptable," Rafael Pardo told reporters.

The crime coincided with the visit to Colombia by a group of UN Security Council ambassadors, who have arrived to express their support for the government's peace process with the FARC.

The kidnapped official, a Colombian identified as Harley Lopez, was abducted Wednesday by members of a dissident faction of the FARC led by alias "Ivan Mordisco" in the southeastern province of Guaviare.

The kidnapping occurred in Barranquillita, a remote hamlet two hours by land from the Guaviare municipality of Miraflores, where the official was working on an illegal crop substitution program.

"The national government rejects this lamentable action," Pardo's office said in a statement, adding that it jeopardizes the crop-substitution program and the physical wellbeing of UNODC (United Nations Office on Drugs and Crime) officials.

Lopez is a member of a commission that is led by Miraflores' mayor and also includes officials from a national government anti-narcotics office, community task forces and UNODC officials, the statement added.

Those officials had been meeting with nearly 400 area coca farmers to increase their awareness about the illegal crop substitution program.

The kidnapping coincided with Wednesday night's arrival of the UN Security Council ambassadors.

Those visitors are to meet on Thursday with Colombian President Juan Manuel Santos and civil society representatives and on Friday are to visit one of the 26 "Temporary Hamlet Zones for Normalization" where some 7,000 FARC members are preparing to leave aside their weapons and demobilize.