It was the deadliest such attack by the ELN since negotiations started with the government of the country's first-ever leftist president, Gustavo Petro, last November.

"We have received preliminary information about the killings of nine of our soldiers" in El Carmen in the North Santander department, the army said in a statement, attributing the attack to the National Liberation Army (ELN).

Petro on Twitter expressed his "total repudiation" of the attack.

He made no reference to the ELN, but said the perpetrators were people "still absolutely far from peace."

Most of the soldiers were young men performing compulsory military service, Petro said.

According to the North Santander regional government, eight other soldiers were injured.

And armed forces commander General Helder Giraldo said the military would conduct "operations in the area against those responsible."

- 'Total peace' -

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.

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

The groups were the ELN, 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 far-right rival Federico Gutierrez said on Wednesday.

A next round of talks with the ELN is 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.