Washington signed a deal with the Taliban in February that required the Afghan government to free 5,000 Taliban prisoners.
The Taliban warned of more violence if the U.S. and the Afghan government continue alleged violations of the deal.
The Taliban have also said they would guarantee the security of health and aid workers traveling to their areas.
The prisoner release is part of a peace deal signed last month by the Taliban and U.S.
Afghanistan's political turmoil has impeded each tentative step toward negotiations with the Taliban.
Since the signing of the deal, the peace process has stalled amid political turmoil in Afghanistan.
An Afghan government official said the releases were delayed because more time was needed to review the list of prisoners.
The four-star commander of U.S. forces in Afghanistan made the stunning admission Tuesday.
Afghan President Ashraf Ghani issued a decree Wednesday promising to release 1,500 Taliban prisoners as a goodwill gesture.
The U.S. has said its troop withdrawal from Afghanistan will be linked to the Taliban keeping their promises.