The department called the announcement "important and necessary," but department spokeswoman Neda Brown told Colombia Reports, "FARC has promised release of prisoners before; such promises are not credible until fully carried out."
Brown said the U.S. government joined Colombian President Juan Manuel Santos in urging FARC, the Spanish initials for the Revolutionary Armed Forces of Columbia, to "renounce all violence and lawlessness."
FARC announced on its Web site Sunday it would release 10 members of the security forces and would no longer kidnap civilians to finance its insurgency.
But it remained unclear whether the group planned to release kidnapping victims already being held for ransom. The number of such victims is unknown but aid organizations suggest there could be more than 700 being held.
Colombia Reports said there is skepticism about whether FARC leaders could enforce the edict. Its rebel fronts are spread widely and recent killings have reduced its leadership.
Former FARC leader Guillermo Saenz was killed last year during a firefight with government troops, and Timoleon Jimenez, also known as Timochenko, replaced him in November.
The government has said FARC must end all kidnappings as a first step toward a peace dialogue the rebels seek.