The official, with the UN Office on Crime and Drugs, was kidnapped Wednesday in the southeastern department of Guaviare.
The official, with the UN Office on Crime and Drugs, was kidnapped Wednesday in the southeastern department of Guaviare while on a tour to promote replacing coca with legal crops, according to the United Nations.
His name and nationality were not given.
Colombian President Juan Manuel Santos said after meeting the UN Security Council ambassadors on Thursday: "I hope he will be with us very quickly."
The UN office in Colombia said "we are working with the authorities for his immediate and secure release."
There was no immediate public confirmation or comment from the leftwing rebel Revolutionary Armed Forces of Colombia (FARC).
But the abduction cast a shadow over the landmark peace deal signed in November between the leftwing rebels and the government that calls for the FARC to transition its 7,000 fighters to civilian life.
Some FARC dissidents have refused to lay down their arms and still maintain links to drug trafficking.
An estimated 300 of these dissident guerrillas are believed to be active in Guaviare, Kyle Johnson, an analyst with the International Crisis Group, said.
Johnson said the visiting UN ambassadors will leave with the impression that "things are not going as well" as the Colombian government had been saying.
Elbio Rosselli, Uruguay's ambassador to the United Nations and a leader of the UN delegation, suggested the kidnapping was intended as a signal to the international community.
"There was an intention behind this cowardly act and it simply won't change the attitude or interest of the UN Security Council in contributing to the process that the Colombians have begun," he told Blu Radio.
Rafael Pardo, a senior adviser to Santos, called the kidnapping "completely unacceptable."
"We hope that he is safe and sound and that they free him soon," he told Radio Caracol.
He added that FARC dissidents had announced they were to free the UN official at midday (1700 GMT) on Thursday. But that hour passed with no notification of a release.
A police source told AFP that an elite anti-kidnapping brigade had gone to the area Guaviare and was coordinating with military forces.
The UN Office on Crime and Drugs is active in Colombia, which is the world's biggest producer of cocaine, with an output of 646 tonnes in 2015.
The peace deal is seen as a way to promote development in areas currently dependent on coca-growing, and to curb drug trafficking.
Under its terms, the FARC agrees to detach itself from the cocaine business that helped finance its five-decade-old armed struggle.
The dissident rebels resisting that will be fought as common criminals, the Colombian government says.
The UN mission overseeing the implementation of the peace accord said that as of this week it has received and securely stored 1,000 weapons handed in by the FARC fighters.