Bogota hopes the talks with the National Liberation Army (ELN) will bring it on board alongside Colombia's biggest rebel force, the Revolutionary Armed Forces of Colombia (FARC), in a bid to end what is seen as the last major armed confrontation in the West.
"If we achieve peace, it will be the end of the guerrillas in Colombia and therefore in Latin America," said the country's President Juan Manuel Santos.
His chief negotiator Frank Pearl and ELN commander Antonio Garcia announced the decision in a joint statement earlier after meeting yesterday in the Venezuelan capital Caracas.
The ELN is a leftist group like the FARC but they have fought as rivals for territory in a many-sided conflict that started as a peasant uprising in 1964.
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While the FARC has observed a ceasefire since last year as its own peace talks have advanced, the ELN has continued attacks.
Accords bringing in the government and the FARC and ELN would establish peace between the main remaining players in the conflict, which over the decades has drawn in right- and left-wing guerrillas, government troops and gangs.
The government and ELN said six other countries will act as guarantors of the peace process: Brazil, Chile, Cuba, Ecuador, Norway and Venezuela.
The FARC's chief negotiator Ivan Marquez called it "a historic moment for Colombia," in a Twitter message.
South American regional bloc UNASUR said in a statement the new negotiations were the "missing piece" of the peace drive. Cuba and Venezuela were among countries that hailed the breakthrough.