ELN commander Uriel announced on social media the "72-hour armed strike in the national territory" from Friday.

The ELN is the last recognized guerrilla movement still fighting authorities in Colombia and can count on around 2,300 fighters.

"All the country's units are on maximum alert to deal with any requirement to protect the population," said Defense Minister Carlos Holmes Trujillo.

He said the armed forces would "respond with the adequate and legitimate firmness, and with all the rigor, in the face of these terrorists' threats."

The ELN has announced armed strikes before but never on a nationwide level.

The group warned citizens against using roads during the strike, adding that "staying in their homes will avoid unfortunate consequences."

The ELN is the last remaining armed guerrilla group operating in Colombia since the FARC laid down weapons in 2016 and formed a political party after signing a peace deal with the government of then-president Juan Manuel Santos.

Yet Colombia remains wracked by a multi-faceted armed conflict involving left-wing guerrillas, right-wing paramilitaries, drug-traffickers and security forces.

Over the last 50 years, eight million people have either been killed, disappeared or displaced due to the ongoing conflict.