A Spanish journalist working in Colombia for almost two decades has disappeared while reporting a story on the illegal drugs industry and is feared kidnapped by Marxist rebels operating in the area, military sources said on Sunday.

Salud Hernandez, a journalist with Spain’s El Mundo and local newspapers, disappeared close to the town of El Tarra in the northern province of Norte de Santander on Saturday.

It is thought that Hernandez has been kidnapped by the National Liberation Army (ELN), the second-biggest rebel group in Colombia, which has continued taking people hostage even while it seeks a peace accord with the government.

"I order priority and dedication from the armed forces in establishing where the journalist Salud Hernandez is," President Juan Manuel Santos said from his Twitter account.

The armed forces have activated a search operation for her in the area, according to a Tweet by the military.

Hernandez is known for her opinion columns that have been highly critical of Colombia's insurgent groups.

Colombia has been in peace talks with the bigger rebel group, the Revolutionary Armed Forces of Colombia (FARC), since the end of 2012 and the ELN recently agreed to begin talks with the government.

Santos has said that no negotiations will start until all hostages are freed by the group.

The 2,000-strong ELN has increased oil pipeline bombings in recent months and continued kidnappings, in what many see as an attempt to pressure the government into talks.

Inspired by Cuba's 1959 revolution, the ELN has battled a dozen Colombian governments since it was founded by radical Catholic priests in 1964.

While many Colombians are suspicious of peace talks with both groups, they are tired of the violence that has killed more than 220,000 and displaced millions over more than half a century.