BOGOTA (Reuters) - A member of a U.N. team working on a project in Colombia to substitute illegal crops was kidnapped by dissident former Revolutionary Armed Forces of Colombia (FARC) rebels who have rejected the peace process, the United Nations and the government said on Thursday.

Arley Lopez, a U.N. official and a Colombian, was seized on Wednesday by gunmen close to Miraflores in southern Guaviare province, where coca cultivation has long formed a key part of Colombia’s cocaine production. He was stopped by armed men in a convoy of vehicles.

The government is seeking to substitute coca - the raw material for cocaine - with legal crops such as coffee, fruits and cacao.

Colombia’s High Commissioner for Post-Conflict Rafael Pardo told Caracol Radio the group had indicated it will release Lopez on Thursday.

The kidnapping came as representatives of the U.N. Security Council were in Colombia to discuss the peace accord signed late last year with the Marxist FARC to end more than five decades of conflict in the Andean country.

The U.N. “condemns the events that happened yesterday in Guaviare and that go against the integrity and rights of a colleague in the United Nations Office on Drugs and Crime and demand his immediate liberation,” the world body said in a statement.

The government also condemned the kidnapping, which it characterized as “regrettable.”

In declarations following the meeting with President Juan Manuel Santos, the U.N. Security Council representatives did not mention Lopez, but offered support for the peace process.

“Colombia has started this process and it is already an irreversible process, there is no turning back and the Security Council will remain by your side as long as it takes,” said British U.N. Ambassador Matthew Rycroft.

While as many as 7,000 fighters agreed to the pact and are in the process of turning in their weapons to the United Nations, several hundred have refused. Those dissidents have formed a new criminal gang and engage in the lucrative drug trafficking that FARC had engaged in for years, the military has said.

The FARC’s leadership has expelled the dissidents.

Opposition politicians, including former President Alvaro Uribe, are angry the peace accord allows former FARC fighters to avoid jail time and participate in national politics. Many Colombians say the government has not done enough to stop other criminal groups and FARC dissidents filling the void and taking over the drugs business.