UNITED NATIONS (AP) — The process of integrating leftist rebels into Colombian society after more than 50 years of war isn't going well and could impact the peace agreement, the U.N.'s deputy human rights chief said Friday.

Andrew Gilmour, who visited Colombia last week, warned that if rebels from the Revolutionary Armed Forces of Colombia, or FARC, don't become part of Colombian society "there is a strong chance that they will go back to something worse."

He told a small group of reporters that he has seen in Africa and elsewhere that when fighters aren't reintegrated "the peace itself is not sustainable."

Gilmour said it was too strong to say that the rebels would return to war, but he said "there's a momentum that can be lost" and they could turn to drug trafficking and other criminal activity.

After years of thorny negotiations, FARC rebels reached an agreement with the government last year to transition into a political party. The peace deal ended more than half a century of war that caused more than 220,000 deaths and displaced nearly 6 million people.

Gilmour said the Colombian government should be saluted for the progress that has been made.

"I think it has been a success but there are very serious challenges," he said, citing the integration of fighters, an increase in the killing of human rights defenders, including activists and community leaders, and the need for accountability for past crimes.

Gilmour said the FARC rebels have demobilized and given up their weapons "but they have not found alternative means of income for them and their families."

He said government institutions are not moving into areas which the FARC controlled for decades, and as the rebels vacate them "there is a danger that unless the state moves in to fill that vacuum then the vacuum gets filled by highly undesirable elements." He cited people involved with illegal mining and other extractive industries and drugs.

During his trip, Gilmour said, he went to the mountains and saw village leaders who said FARC fighters were coming back "but we have nothing for them."

The fighters come from rural areas, he said, but there were no seeds and tools.

Finding economic opportunities for perhaps 10,000 fighters "should not be beyond the bounds of what is basically a developed country and economy," Gilmour said.