The U.S. lawmakers, including Republican Sens. John McCain of Arizona and Lindsey Graham of South Carolina, said during a Kabul meeting with Karzai freeing the prisoners without trials would seriously damage Afghan-U.S. relations, Voice of America reported.
Plans are to release the prisoners from the Bagram Prison near Kabul, a U.S. prison before it was handed over to the Afghan government last year.
Graham said releasing the 88 dangerous detainees would violate existing agreements to deal with them, VOA reported.
"There is much evidence to suggest a wrongdoing," Graham said. "Over 60 coalition forces have been killed as a result of actions by these 88 and 57 Afghans have been killed by the actions of these 88. If this release goes forward, it would be undercutting the [Afghan] rule of law, it would be a major step backward and it would have an unbelievably negative impact on the future relationship between the American people and the Afghan government."
The prisoners were captured by Afghan and U.S.-led forces as part of their counter-terrorism efforts, the report said.
Karzai has set up a special review committee to investigate and determine the fate of prisoners and 500 detainees have already been released.
The senators said they told Karzai the prisoners are members of the Taliban or related militant groups.
The New York Times said without the support of Graham and McCain any hope of keeping a residual U.S. force in Afghanistan after this year would dwindle.
Karzai has been stalling to sign a security agreement that will decide how many U.S. troops would remain after combat operations in Afghanistan end by the close of this year.
McCain said he told Karzai that a recent poll showed only 17 percent of Americans support the war.
Karzai, after his meeting with the U.S. senators, was quoted as saying: "Afghanistan wants the innocent prisoners to be released based on Afghan laws and criminals must be punished."