An employee of the United Nations has been kidnapped by a dissident rebel faction in southern Colombia, marring a visit by UN Security Council ambassadors to show support for the South American nation's recent peace deal.

President Juan Manuel Santos' administration said that the Colombian national was working on a crop substitution project in the southern state of Guaviare when he was taken captive late on Wednesday.

Rodrigo Pardo, the president's top aide for post-conflict planning, said the captors are a unit of the Revolutionary Armed Forces of Colombia who refused to lay down their weapons as part of a peace deal last year.

"These people live thanks to drugs and they want to continue living that way," Pardo said, adding that the incident would not stop the government in its goal of eradicating cocaine crops in areas once dominated by the FARC.

A group of UN officials in coordination with Colombian authorities was attempting to negotiate the worker's release.

"Obviously the situation when someone's been kidnapped, the less we say the better," UN spokesman Stephane Dujarric said when asked about the hostage.

"What I will tell you is that we're obviously in touch with the Colombian authorities to help secure the person's release."

Peace process going slow

Santos met on Thursday with ambassadors to the UN Security Council who are in the country to demonstrate their commitment to ending a half-century war that has caused more than 220,000 deaths and displaced nearly six million people.

Al Jazeera's Alessandro Rampietti, reporting from Bogota, called it a "historic event" as it is the first time the whole Security Council has travelled to a Latin American country.

"It comes at a time when the peace process is suffering delays in its implementation and also amid a return to violence in some areas that were once under FARC control," Rampietti said.

The 15 ambassadors will meet Colombian politicians, members of civil society and victims of the conflict, and also travel to one of the FARC disarmament zones.

In January, the Colombian government and the FARC jointly asked the United Nations to monitor any ceasefire and disarmament process - a rare request to the UN for help.

Currently, some 450 UN observers are spread out over more than 20 rebel camps nationwide, overseeing the laying down of weapons by the guerrillas.

But the process is slow and has run into logistical hurdles as well as delays by the government in approving and implementing legislation mandated by the peace deal.

FARC leader Ivan Marquez said in an interview this week with Red Mas Noticias that if needed, the two sides would extend the May 31 deadline to complete the demobilisation process, adding that the rebels will not begin turning over their arms until the government follows through on its commitment to free hundreds of jailed rebels subject to an amnesty.

British Ambassador Matthew Rycroft, who is co-leading the visit, said the UN would remain in Colombia as long as it takes to secure peace.