The first round of a presidential election will be held on May 27 following parliamentary polls last month won by right-wing parties opposed to the peace deal -- though they fell short of a majority.

UN envoy Jean Arnault told the Security Council that there must be "a sustained drive to put the reintegration of former combatants on a more solid basis before the end of the government's mandate."

Under the peace accord, the Revolutionary Armed Forces of Colombia (FARC) disarmed its 7,000 fighters in order to join the political process and return to civilian life.

Arnault warned that a weak drive to bring the ex-combatants back to the mainstream "can only increase the risk of the drift of some ex-combatants to criminal groups," such as those who have recently waged attacks on the border with Ecuador.

Colombian President Juan Manuel Santos, who signed the peace agreement with the FARC in November 2016, is stepping down in August after two terms.

A presidential runoff is planned for June 17.