President Gustavo Petro, Colombia's first-ever leftist president, who has said he wants "total peace" in the country, announced the truce on Twitter.

"We have agreed to a bilateral ceasefire with the ELN, the Second Marquetalia, the Central General Staff, the AGC and the Self-Defense Forces of the Sierra Nevada from January 1 to June 30, 2023," Petro tweeted on New Year's Eve.

The ceasefire was declared "extendable depending on progress in the negotiations."

Despite the peace pact that saw FARC guerrillas disarm in 2017, armed groups remain locked in deadly disputes over drug trafficking revenues and other illegal businesses, according to the Institute for Development and Peace Studies (Indepaz), an independent think tank.

Colombia is the world's largest cocaine producer.

The government, in a statement Sunday, said the ceasefire would be monitored by the United Nations, Colombia's human rights ombudsman and the Catholic Church.

Peace talks had been suspended under the government of Ivan Duque (2018-2022), but Petro resumed negotiations soon after taking office on August 7.

So far, negotiations with various armed groups -- with a combined total of more than 10,000 fighters -- have failed to end a spiral of violence engulfing the country.

Indepaz recorded nearly 100 massacres last year.

- 50 years of conflict -

The National Liberation Army (ELN), the last recognized insurgency in the country, has been negotiating with the government since November.

On December 19 it announced a unilateral ceasefire until January 2.

The government then called on other groups to join the truce.

The Segunda Marquetalia and Central General Staff -- splinter factions of FARC that rejected the 2016 peace pact -- have held separate exploratory talks with the government.

AGC, the country's largest drug gang, is made up of remnants of extreme right-wing paramilitaries that demobilized in the early 2000s during the presidency of Alvaro Uribe.

The government is offering the groups "benevolent treatment from the judicial point of view" in exchange for "a surrender of assets, a dismantling of these organizations and the possibility that they stop exercising these illicit economies," Senator Ivan Cepeda recently told AFP.

The UN secretary-general's special representative in Colombia, Carlos Ruiz Massieu, on Sunday saluted Petro's announcement.

"The United Nations supports all efforts aimed at reducing violence in the territories, protecting the communities affected by the conflict and building peace in Colombia," he said on Twitter.

The office of Colombia's rights ombudsman, for its part, said it would be vigilant in verifying "that the cessation of armed actions by illegal armed groups is complied with."

Some dissidents refused to lay down their arms alongside thee FARC when the fearsome rebel army signed an agreement to end more than five decades of conflict.

Colombia has suffered more than 50 years of armed conflict between the state and various groups of left-wing guerrillas, right-wing paramilitaries and drug traffickers.

There are still about 90 political and criminal groups operating in the country, according to Indepaz.

When Petro took power last August, he vowed to negotiate with all Colombian armed groups as part of his "total peace" policy.

But Uribe's right-wing Democratic Center party has dismissed the "total peace" approach as "defending crime and impunity."

The ceasefire was "an unacceptable capitulation of the state before the illegal armed groups," Democratic Center congressman Andres Forero tweeted on Sunday.