BOGOTA, Colombia (AP) — Militants in Colombia killed nine soldiers and wounded nine others in an attack early Wednesday on a military unit securing a northern pipeline, officials said, complicating efforts by the country’s new leftist president to negotiate a lasting peace.

The assailants attacked with long-range weapons and improvised explosive devices in El Carmen, in Norte de Santander state near the Venezuelan border, and the military’s preliminary assessment was that the attackers were from the National Liberation Army, or ELN.

It would be the deadliest attack by the guerrillas since their resumption in November of peace negotiations with the government, and the violence undermines efforts by President Gustavo Petro to bring “total peace” to the nation of 50 million people.

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.

The president called for consultations with government negotiators and international partners in the peace process with the guerrillas. “A peace process must be serious and responsible with Colombian society,” Petro said.

Otty Patiño, head of the government’s peace delegation, said in a statement that he would ask Petro to make a ceasefire a condition for civil society to participate in the peace process.

In addition to the attack, Patiño said, the ELN has been increasingly harassing civilians in the departments of Cauca, Arauca, Chocó and Nariño, engaging in a “flagrant violation of the norms of International humanitarian law that they claim to abide by.”

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

Camilo González Posso, president of the Colombian nongovernmental group Institute for Development and Peace Studies (Indepaz), called the attack “a tremendous political mistake by the ELN,” given that the group previously has indicated an openness toward negotiating with Petro’s administration.

“At the table it says that the situation has changed when it comes to a progressive government, and on the ground it attacks,” Posso said.

The nine soldiers wounded in Wednesday’s attack were taken to hospitals in the Norte de Santander capital of Cúcuta.

The commander of Colombia’s military, Major General Helder Giraldo, said the unit was attacked while securing the Caño Limón-Coveñas pipeline of state oil company Ecopetrol and was not involved in offensive operations. Ecopetrol facilities have faced frequent attacks by ELN forces in the past.

Giraldo said the military would continue operations in the area against the ELN and would file complaints over the “serious violation of human rights and international humanitarian law.”

Founded in 1964 and originally inspired by the Cuban revolution, the ELN now has around 2,000 to 4,000 troops in Colombia and neighbouring Venezuela. Human human rights groups 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.