The Colombian military has killed seven leftist guerrillas and captured an eighth in an operation near the Venezuelan border, President Juan Manuel Santos said Saturday amid a sharpening conflict with the country's second-largest guerrilla group.

Santos said the guerrillas belonged to the National Liberation Army (ELN), which has been in preliminary discussions for two years with the government about joining formal peace talks.

Tensions have risen sharply, however, since February 3 when the group captured a Colombian soldier.

"If they want to begin some kind of public conversation, they have to free those two kidnapped people they are holding," Santos said, referring to the captured soldier and a civilian who has been in rebel hands since 2014.

"There will be no step toward a negotiation if they do not at a minimum fulfill these conditions," he said in a speech to the leadership of the National Police.

The seven ELN guerrillas were killed Friday night in Arauca, a region that borders Venezuela, Santos said.

The operation followed a three-day ELN offensive aimed at disrupting traffic on major roads. Three police were killed and dozens of violent incidents were reported across the country from last Sunday to Tuesday.

The army had put itself on "maximum alert" in anticipation of more violence on the 50th anniversary, last Monday, of the death of Camilo Torres, a rebel priest who was an early leader of the ELN.

Founded in 1964, the ELN is the country's second-largest guerrilla group, with an estimated 1,500 fighters.

Colombia's largest guerrilla group is the FARC, or Revolutionary Armed Forces of Colombia, which has been in formal peace talks since November 2012.

The FARC and the Santos government have pledged to reach a final peace agreement by March 23, but negotiators have yet to agree on details of a disarmament process and the mechanism by which a final accord will be ratified.

Colombia peace drive wobbles, president says time short
Bogota (AFP) Feb 19, 2016 - Colombia's president Friday pressured FARC rebels to respect a March 23 deadline for a peace deal to end a half-century conflict, warning time was short as tensions rose over the negotiations.

Progress made in recent weeks at peace talks in Havana looked in peril as President Juan Manuel Santos and leaders of the Marxist guerrilla force squabbled over restrictions on the rebel leaders' movements in Colombia.

Under pressure after FARC members provoked outrage by appearing at a political rally, Santos warned time was running out for the two sides to reach agreement on the final points of the peace deal.

Those points "must be resolved before the agreed date. If that does not happen, Colombians will understand that the FARC were not ready for peace," Santos said in a speech on Friday.

"The time for finishing the negotiations is running out. The date of March 23 -- agreed on by the president and the leader of the FARC -- is less than five weeks away."

Santos on Thursday suspended visits by FARC negotiators to their members in Colombia.

He said they had breached an earlier agreement by attending a political rally with members of the public.

The FARC in a statement branded that an "unjustified controversy."

"This moment, in which the possibility of signing a definitive peace agreement is becoming an increasing certainty, requires that... the main players in the process show they are determined to negotiate together whatever obstacles may stand in the way of a political solution."

The two sides have yet to agree on the details of disarmament and the mechanism by which the final accord will be ratified.

Santos has vowed to put the peace deal to a popular vote in Colombia but the FARC wants it passed by a constituent assembly.

The United Nations has agreed to send a political mission of unarmed observers to monitor disarmament and the transition to peace.

The FARC launched in the aftermath of a peasant uprising in 1964 and authorities estimate it currently has some 7,000 members.

The Colombian conflict has drawn in right-wing paramilitaries, drug traffickers and several leftist rebel groups. The war has left more than 220,000 people dead.