Colombian President Juan Manuel Santos and National Liberation Army (ELN) negotiators announced the ceasefire two days before a visit to Colombia by Pope Francis.
ELN chief negotiator Pablo Beltran said his force’s ceasefire was “the first miracle of the pope’s visit.”
The ceasefire follows a separate accord that saw the disarmament last month of Colombia’s biggest rebel group, the FARC.
Santos said it was “great news that we are sure will delight” the Argentine pontiff.
Under the ceasefire, “there will be an end to kidnappings, attacks on oil pipelines and other hostilities against the civilian population” by the ELN, Santos said in a televised address.
The government for its part promised to improve conditions for imprisoned ELN members and protect civil leaders from attacks in the conflict zone.
– Toward ‘complete peace’ –
The 1,500-strong ELN has been in negotiations with the government since February.
Santos said the ceasefire will be renewed depending on progress on details still to be thrashed out with the leftist rebels.
“It will come into effect on October 1, initially for 102 days, that is to say until January 12 of next year.”
The ELN delegation earlier announced the deal on Twitter.
“When the days of celebration during Francis’s visit to Colombia are over, we will continue, determined to advance toward a de-escalation of the conflict until complete peace becomes a reality.”
Francis is scheduled to tour Colombia from September 6 to 10.
The two delegations to the talks said at a news conference in Quito that the Colombian government, ELN, UN and the Catholic church would jointly monitor the fulfillment of the ceasefire.
“It is a challenge for us to fulfill all the aspects agreed upon in this ceasefire. We hope to keep our word,” said Beltran.
– Tricky talks –
The 7,000 members of the FARC finished disarming last month under UN supervision, despite resistance to the deal from critics who said the FARC got off too lightly.
Analysts warn that the talks with the ELN, under way since February, risk being even more complicated than the four-year negotiations with the FARC.
The deal that came out of those talks was considered to have practically ended the conflict, but other risks remain.
The rebel group has a looser command structure than the FARC had.
“The lack of cohesion in the ELN is a big difference compared with the FARC,” said Camilo Echandia, a conflict analyst from Colombia’s Externado University.
– ‘Fragile’ ceasefire –
Officials meanwhile say remnants of right-wing paramilitary groups are still fighting the ELN for control of the drug trade.
Authorities have also reported deadly attacks by ELN fighters against state forces.
“We have to be very realistic: this ceasefire is going to be quite fragile,” warned Frederic Masse, another analyst at Externado University.
“It could even be counter-productive and, instead of driving forward the negotiations, end up freezing them.”
The FARC and ELN formed in 1964 to fight for land rights and to protect poor rural communities.
The conflict drew in leftist guerrillas, right-wing paramilitary groups and state forces.
It left 260,000 people confirmed dead, more than 60,000 missing and seven million displaced.
– Pope effect –
Both sides had said they hoped to agree to a temporary ceasefire before the pope’s arrival.
“The visit of Pope Francis should provide extra motivation to speed up the search for an agreement,” the ELN said on Twitter.
It added that the peace talks aimed above all to help poor rural communities suffering “the unfortunate consequences of the conflict.”
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