Gustavo Petro in a Tweet said the perpetrators of the attack were people "still absolutely far from peace," five months into negotiations with the ELN.

Officials said ELN guerrillas killed nine soldiers in an attack with explosives and automatic gunfire in El Carmen in the North Santander department.

It was the deadliest such attack by the ELN since peace negotiations started last November.

The country's first-ever leftist president issued a "total repudiation" of the attack and called government negotiators for "a consultation," along with guarantor countries.

"A peace process must be serious and responsible towards Colombian society," the president said.

Defense Minister Ivan Velasquez said Petro called the talks "to examine this truly serious event" he qualified as "an attack on peace."

Ahead of the meeting with Petro, Colombian peace commissioner Danilo Rueda would not rule out suspending negotiations after the events he said "shun the construction of peace."

"Healthy decisions will be made for the country," he told reporters.

- 'Total peace' -

Army chief Helder Giraldo said the soldiers had been patrolling one of the country's largest oil pipelines, frequently targeted by the ELN.

They had not been deployed in an offensive capacity.

Most of the soldiers were young men in their 20s performing compulsory military service, officials said.

Giraldo said the military would conduct "operations in the area against those responsible."

Colombia has suffered more than half a century of armed conflict between the state and various groups of left-wing guerrillas, right-wing paramilitaries and drug traffickers.

Fighting has continued despite a peace deal that saw the FARC guerrilla group disarm in 2017.

With armed groups disputing lucrative drug trafficking revenues and other illegal businesses, the Indepaz research institute reported nearly 100 massacres in Colombia last year.

The government resumed peace talks with the ELN, considered the last active guerilla group in Colombia, after Petro took office last August pledging to bring a "total peace" to the violence-riddled country.

Talks with the ELN had been suspended by Petro's conservative predecessor Ivan Duque following a car bomb attack on a police academy in Bogota in 2019 that left 22 people dead.

The ELN has taken part in failed negotiations with Colombia's last five presidents.

- 'Surrender' -

On New Year's Eve, Petro announced a truce had been agreed with the country's five largest armed groups, including the ELN, from January 1 to June 30.

The others were two dissident splinter factions of the now-disbanded FARC, the Gulf Clan narco group and the Self-Defense Forces of the Sierra Nevada, a rightwing paramilitary organization.

The ELN, however, immediately refuted any such deal, forcing the government to backtrack.

Hostilities continued and the army has reported several attacks on its members by the ELN.

Last September, FARC dissidents killed seven police officers in the central department of Huila.

Several ELN fighters have also been killed and captured in military operations in recent months.

Earlier this month, the government suspended its truce with the Gulf Clan -- the country's largest drug cartel -- over attacks on civilians and uniformed personnel.

Colombia's opposition frequently criticizes the president for concessions he is willing to make for peace.

"'Total peace' cannot mean surrender of the country" to criminal groups, Petro's defeated rightwing rival Federico Gutierrez said on Wednesday.

Talks with the ELN had been held in Caracas and Mexico City, with another round due to be held in Cuba, though a date has yet to be announced.

Earlier this month, the two parties agreed to hold ceasefire talks as well.

Founded in 1964 by trade unionists and students inspired by Marxist revolutionary icon Ernesto "Che" Guevara and the Cuban revolution, the ELN counts on about 3,500 fighters.