In an internet article posted on Saturday, Castro said he was against kidnapping and holding prisoners in the jungle.
But he also said the Revolutionary Armed Forces of Colombia (Farc) should not lay down their weapons, warning that as a rule, fighters who have done so over the past half-century "did not survive to see the peace".
"If I may dare to suggest something to the Farc guerrillas, it is that they simply, by whatever means at their disposal, declare that they have unconditionally freed all the hostages and prisoners still under their control," Castro wrote in the article just days after Colombia's military rescued the group's most prized hostages.
Colombia's government says the Farc still holds about 700 hostages for political leverage or ransom, although its most valuable captives were among the 15 rescued last week, including Betancourt and three American defence contractors.
Words of hope
In Colombia on Sunday, five of the recently rescued hostages went on the country's airwaves to speak about their ordeal and encourage the remaining hostages not to give up hope.
Betancourt also joined the Voices of Kidnapping radio show, calling in from France where she says she will remain for a while.
The programme on Colombian station Caracol Radio offers an emotional lifeline to rebel-held hostages, who with portable radios often hear messages from their families.
The rescue operation last week in which the military tricked the rebels into releasing the 15 hostages, has boosted the popularity of Alvaro Uribe, Colombia's president.
His approval rating went up nearly 20 per cent to 91 per cent, according to the latest opinion poll in the country.
Plea to remember
But families whose loved ones are still being held by the rebels are calling for him to do more.
The mother of one such hostage, Magdalena Rivas Hernandez, says she fears her government will forget lower-profile captives such as her son, Lieutenant Elkin Hernandez Rivas, who was kidnapped in October 1998.
"Now that the high profile hostages like Ingrid Betancourt and the three Americans have been rescued, I’m worried that the Colombian government will forget about every day people like my son," she told Al Jazeera.
"The president has to do something to help my son and the other people that remain prisoners in the jungle."