BOGOTA, Colombia (AP) — Colombia's largest remaining rebel group has acknowledged its role in the death of an indigenous leader in violation of a month-old cease-fire, raising tensions in peace talks with the government that are already mired in doubt.

President Juan Manuel Santos' government on Monday condemned the killing of Aulio Isarama and demanded the National Liberation Army provide information to lead to the arrest of those responsible. It also called on the group known as the ELN to take concrete steps to guarantee such incidents aren't repeated.

Isarama, a member of the Embera people, was killed last week on the reservation in the western state of Choco, where he was governor. He was the 11th indigenous leader slain this year in Colombia, according to human rights groups.

The ELN's western front said in a statement that it had detained Isarama for allegedly being an informant for military intelligence. It said his death occurred after he physically assaulted one of the ELN rebels who had been escorting him to where he was to be interrogated.

"We're committed to carry out an internal reflection at all levels so that these incidents don't happen again," the group said in a statement.

While one of Santos' top aides has questioned the wisdom of continuing talks with the ELN (probably need to name him or give brief quote), the government has so far indicated it will continue.

Peace Commissioner Rodrigo Rivera said that under the protocols agreed to as part of last month's cease-fire, no single incident is ground for either side walking away from the negotiating table. A committee of observers that includes representatives of the United Nations will now investigate the death of Isarama and recommend steps to negotiators so there are no further such incidents.

After years of exploratory talks, peace negotiations with the ELN began in February following an agreement with the much-larger Revolutionary Armed Forces of Colombia. But progress has been slow and many question whether the ELN's top commanders have effective control over their estimated 1,500 troops, many of whom in the past have carried out kidnapping and bombings of oil pipelines that have complicated efforts at negotiating an end to the half-century conflict.