Colombians head to the ballot box on May 29 for a first election round with leftist former guerrilla Gustavo Petro favored to win.

"We are declaring a ceasefire from 0:00 on May 25 until 24:00 on June 3, so that those that want to vote can do so in peace," the Marxist National Liberation Army (ELN) said in a statement.

The ceasefire would include a halt to actions against Colombia's security forces but the ELN added: "we reserve the right to defend ourselves if attacked."

Defense Minister Diego Molano, however, said the ELN's move was inspired purely by a wish to "position itself for future dialogue" with the new government.

"Security (in Colombia) is assured by the security forces," he insisted.

After the Revolutionary Armed Forces of Colombia (FARC) signed a peace deal with the state in 2016, the ELN became the violence-ridden South American country's last rebel group.

Former president Juan Manuel Santos had been in negotiations with the ELN to also make peace but his right-wing successor Ivan Duque put an end to the talks following a car bomb attack on a police academy in Bogota claimed by the Marxist guerrillas that killed at least 20 people.

The ELN had also called a six-day truce in March during legislative elections and presidential primaries.

Formed in 1964 in the wake of the Cuban communist revolution, the ELN counts about 2,500 fighters and an extensive support network in urban centers, mostly on the border with Venezuela and along the Pacific coast.

It is largely funded through drug trafficking, and continues fighting over territory and resources in parts of Colombia with FARC dissidents, rightwing paramilitary forces and narco cartels.

Following the election, Petro is expected to face a run-off vote on June 19 against right-wing candidate Federico Gutierrez.

Petro was formerly a member of the 19th of April Movement (M-19), an urban left-wing nationalist guerrilla group that laid down arms in 1990.

After that he entered politics and served as mayor of Bogota from 2012 to 2015, before becoming a senator.

Petro has vowed to negotiate with rebels if elected president, whereas Gutierrez has promised a frontal assault against the ELN.

Colombia is the world's largest producer and exporter of cocaine.