Colombia's Marxist FARC rebels agreed Sunday to remove child soldiers from their ranks as part of the peace deal they are close to signing with the government.

The Revolutionary Armed Forces of Colombia (FARC) agreed to "implement departure of those under 15 years of age as soon as terms are agreed upon," the parties said in a statement from Cuba, which is hosting peace negotiations.

Terms must still be set on how the children will be transitioned and reintegrated back into society.

Talks remain underway over the fate of FARC members between the ages of 15 and 18, as well as those under the age of 15 involved in war crimes, the statement said.

News of the agreement comes as the FARC and representatives of the Colombian government of President Juan Manuel Santos negotiate terms of a definitive ceasefire, one of the last major points before a peace deal can be struck to end Latin America's longest-running civil war.

"We reached a historic agreement in Havana to get children out of the war," Santos wrote on Twitter, adding that the youths would be sent back to their families.

Lead government negotiator Humberto de la Calle said the agreement was "a crucial advance in the process of putting a full stop to the war."

- How many child soldiers? -

The two sides did not immediately state how many minors would be exiting the FARC's ranks.

The 7,000-strong FARC said that they never wanted to recruit children to fight, but did accept into their ranks orphans and victims of domestic abuse.

Many of the group's adult fighters joined when they were minors.

The guerrillas said separately that 21 FARC members under the age of 15 had already been released.

The FARC announced in February 2015 that it had stopped accepting recruits under the age of 17, and a year later said it had banned all recruits under the age of 18.

According to government figures, authorities have taken some 6,000 children from illegal armed groups over the past 17 years, more than half of them coming from the FARC.

Present at the announcement was Algerian-born Leila Zerrougui, UN Secretary General Ban Ki-moon's special representative for Children and Armed Conflict.

Zerrougui urged the FARC to "begin as soon as possible to implement" the agreement and release the children.

The Colombian conflict has drawn in several leftist rebel groups, right-wing paramilitaries and drug gangs.

It has left 260,000 people dead and 45,000 missing since breaking out in 1964, while another 6.6 million have been uprooted. Human rights groups say atrocities have been committed on all sides.

Colombia's Interior Minister Juan Fernando Cristo said Wednesday that peace talks were in the "home stretch," and that Colombia plans to hold a referendum on the deal by September.