A mayoral candidate in southwestern Colombia was killed along with five other people in an attack likely perpetrated by dissidents from the demobilised FARC guerrilla group, the government said on Monday.

The killing was the first of a candidate for October local and regional elections and came days after some prominent former members of the rebel group said they were rearming.

A vehicle carrying Karina Garcia, a Liberal Party candidate in the municipality of Suarez in Cauca province, and the other victims were shot at while it traversed a highway in the mountainous region, before being set on fire.

The other five victims were Garcia's mother, three local activists and a candidate for the municipal council.

The area contains important drug production and trafficking routes, and dissidents - who reject the 2016 peace accord between the government and the Revolutionary Armed Forces of Colombia (FARC) - are known to operate there.

"With sadness, we mention the candidate for the Liberal Party, it is confirmed that alias Mayinbu, who is the leader of a dissident group of the FARC ... is responsible," peace commissioner Miguel Ceballos told reporters.

"We presume he is responsible, because of his influence in the area, of this atrocious crime," Ceballos said.

In a statement, the Defence Ministry offered a reward of nearly US$44,000 for information leading to the capture of two dissident leaders in the province.

Neither Ceballos nor the ministry offered details about proof that dissidents were responsible.

Last week, several former commanders, including two who had been slated to represent the FARC in Congress as part of its new incarnation as a political party, announced a new offensive in a 32-minute YouTube video, because of what they said was the failure of the government to comply with the deal.

The announcement drew condemnation from the government, the United Nations and the FARC political party, whose leadership said the majority of ex-rebels remained committed to the accord despite "difficulties and dangers."

Some 13,000 FARC members, including 7,000 combatants, laid down their arms under the terms of the deal, in exchange for legal amnesty for many, temporary economic support and the chance for the group to become a legal political party.

Although the killings of some former rebels and delays in government funding for economic efforts have sparked criticism of the government, many former fighters say they want to hold up their part in the deal.

- Reuters