The Colombian government said it will announce Wednesday the start of peace negotiations with the country's second-biggest guerrilla group, the left-wing ELN, broadening the push to end a half-century conflict.

The Colombian presidency's High Commission for Peace said in a statement that the government and the left-wing National Liberation Army (ELN) would announce the move in the Venezuelan capital Caracas.

That would open a new front in peace negotiations as the government also closes in on a deal with the country's biggest guerrilla group, the Revolutionary Armed Forces of Colombia (FARC).

The complex conflict between right- and left-wing guerrillas, government troops and gangs in Colombia is considered the last major armed confrontation in the Western Hemisphere. Colombia's Juan Manuel Santos has staked his presidency on ending it.

Official sources told AFP Wednesday's announcement would be attended by the government's top delegate for the existing FARC peace talks, Frank Pearl.

They said they expected two ELN commanders to attend, including Eliecer Herlinton Chamorro Acosta, better known by his nom de guerre Antonio Garcia.

Santos's government has been discussing for more than two years the possibility of launching formal negotiations with the ELN. Those discussions have taken place mainly in Ecuador.

The official sources added that the government and ELN had agreed to let six other countries act as guarantors of the peace process: Brazil, Chile, Cuba, Ecuador, Norway and Venezuela.

For the past three years, the government has been holding talks in Havana with the FARC. The two sides say they have made substantial progress over recent months.

They had aimed to sign a peace agreement on March 23 but that deadline passed with no deal as key issues have not yet been resolved, including disarmament.

In September, the two sides agreed the Marxist rebels would begin disarming "at the latest 60 days after the signing of the final accord."

But they did not set a date to finish disarming.

US Secretary of State John Kerry met with representatives of Colombia and the FARC in Havana last week. He urged them to "redouble" efforts for an end to the conflict.

- War crimes charges -

Starting as a peasant uprising in the 1960s and drawing in various armed groups and gangs, the conflict has killed more than 260,000 people, uprooted 6.6 million people and left a further 45,000 missing.

The United Nations children's agency said this month that hostilities in Colombia have displaced or otherwise disrupted the lives of more than 250,000 children even since the FARC peace talks began three years ago.

Inspired by the Cuban revolution, the ELN was founded in 1964, the same year the FARC launched its uprising. Officials estimate the ELN currently has some 1,500 members and the FARC about 7,000.

Though the prospect of peace with the FARC has raised hopes in Colombia, efforts to bring the ELN into the process are complicated.

As the talks with the FARC have advanced, tensions have remained high over the ELN, which has continued committing attacks and abductions.

Unlike the FARC did last year, the ELN has not yet ordered a ceasefire by its troops, who live in hiding.

The ELN has recently released certain hostages, including a Colombian soldier it had held for more than six months.

But on Tuesday, Colombian state prosecutors called for ELN leaders to be indicted on war crimes charges for attacks against oil installations that they say cost the country billions of dollars.

That case targets leaders including the group's top commander Nicolas Rodriguez Bautista, known as Gabino. Official sources would not rule out that he might attend Wednesday's announcement.