Bogota  - Colombia's leftist rebels are to set free on Sunday two of six captives who are part of a previously announced unilateral hostage release, an opposition senator said.

The Revolutionary Armed Forces of Colombia (FARC) will release four other hostages, on two later occasions, said Senator Piedad Cordoba of the opposition Liberal Party on Wednesday.

The politician, who already last year mediated the release of FARC hostages, did not specify who the remaining captives are and when they will gain their freedom, saying she was not authorized to provide this information.

FARC said on December 21 that it was ready to unilaterally release the former governor of Meta province, Alan Jara, three kidnapped police officers, one military officer as well as Sigifredo Lopez a former member of the provincial parliament of Valle del Cauca.

The six hostages are part of a group of 28 kidnapped lawmakers, police and army offices which FARC wants to exchange for about 500 imprisoned rebels.

It would be the first time the marxist rebels are to set free hostages, since Colombian security forces freed former presidential candidate Ingrid Betancourt and 14 other hostages in a sting operation.

FARC has been engaged in a conflict with the government for decades and is believed to hold up to 700 people captive

A hoped-for exchange in July that would have swapped 50 high-profile hostages for 500 imprisoned guerillas fell through after Betancourt's rescue. Since then, FARC has taken another 20 hostages.

The rebels and the government of conservative President Alvaro Uribe could never agree to a framework for a prisoner exchange.

Brasil announced it would provide helicopters for facilitating the release, while Cordoba thanked the Colombian government for guaranteeing the safety of all involved. (dpa)