Militants in Colombia killed at least nine soldiers and injured another nine in an attack early Wednesday on a military post in a troubled region near the country’s border with Venezuela, authorities said.

The military gave no details of the assault in El Carmen in the state of Norte de Santander, but said a preliminary assessment indicated it was carried out by the National Liberation Army, or ELN, the country’s last active guerrilla group.

It would be the most serious attack by the guerillas since November when they resumed peace talks with the government, and would complicate efforts by Colombia’s first leftist president, Gustavo Petro, to bring "total peace" to the nation of 50 million.

Petro condemned the attack, and said in a tweet that those who carried it out were "absolutely far from peace and the people." He said seven of those killed were soldiers doing compulsory military service and that two were officers.

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The commander of Colombia’s military, Maj. Gen. Helder Giraldo, said the military would continue operations in the area against the ELN and would file a complaint over the "serious violation of human rights and international humanitarian law."

Some rural areas of Colombia are still under the grip of drug gangs and rebel groups despite a historic peace deal in 2016 with the larger Revolutionary Armed Forces of Colombia, or FARC.

Founded in 1964 and originally inspired by the Cuban revolution, the ELN now has around 2,000 to 4,000 troops in Colombia and neighboring Venezuela. Human rights organizations have reported that the group runs drug trafficking routes and illegal gold mines.

ELN attacks have scuttled peace talks in the past. In 2019, a car bombing claimed by ELN that killed 22 people at a police school in Bogota led the government of then-President Iván Duque to suspend all dialogue.