Colombia's Defense Minister Luis Carlos Villegas address a news conference in Bogota, Colombia, on Oct. 26, 2015. Colombia's second-largest rebel group, the ELN, killed 12 members of the security forces in the central province of Boyaca on Monday, the government said, as the troops transported votes from the country's regional elections. REUTERS/John Vizcaino (John Vizcaino/Reuters)

Eleven Colombian soldiers and a police officer were killed in a brazen ambush Monday by the country's second-largest leftist guerrilla force, known as the ELN, dealing a major blow to hopes for government peace negotiations with the group.

The attack was carried out a day after local and municipal elections were held across Colombia. The troops were escorting poll workers and 130 ballots to be counted from remote indigenous villages in the mountains of Boyaca state.

Three soldiers were wounded in the attack, and two soldiers, a police officer, two election officials and their guide are missing, according to Colombia’s Defense Ministry, meaning they have probably been kidnapped.

“This attack shows that the ELN does not understand that this is a time for peace, not a time for war,” Colombian President Juan Manuel Santos said, in a statement he read on television Monday evening “with a broken heart.”

“If the ELN thinks that these actions will help raise its political profile and strengthen its bargaining position, it is totally mistaken,” said Santos, adding that he has ordered the military to intensify strikes against the group in response to the ambush.

The death toll is one of the worst in recent years for Colombian security forces, shattering what has otherwise been a period of relative calm. The government and Colombia’s larger and more powerful FARC rebel group are in advanced stages of peace talks taking place in Cuba that would end their 50-year conflict.

An unofficial cease-fire between the FARC, or the Revolutionary Armed Forces of Colombia, and the government has taken hold in recent months, bringing rural violence to its lowest levels in decades. The sides have set March as a deadline for their agreement to be finalized and for the demobilization of the group’s 7,000 or so fighters to begin.

As talks with the FARC have advanced despite widespread skepticism from the Colombian public, speculation has increased that Santos was close to entering into talks with the ELN, or the National Liberation Army, whose forces are thought to number fewer than 2,000 fighters.

If Santos settles with the FARC but those ELN fighters remain active in Colombia’s mountains and jungles, they could attract disgruntled FARC members who reject the peace deal and refuse to put down their weapons.

The threat of ongoing ELN violence after a potential truce with the FARC would also undermine Santos’s broader effort to pacify the country and its lawless rural areas. Many Colombians are wary of his overtures to the groups, accusing him of going too easy on the guerrillas.

But the 12 killings and apparent hostage-taking by the ELN appear to have dashed hopes that talks with the group would get underway soon.

“For the ELN and all of those who don’t accept the path to peace, the message is clear,” Santos said. “We will confront them with the full weight and power of our armed forces.”

FARC units killed 11 soldiers in an ambush in April, producing a short-term crisis in the negotiations that began officially in 2012. But talks quickly resumed and the FARC announced a unilateral cease-fire in July that has essentially held since then.