Sens. John McCain and Lindsey Graham visited the Afghan president on Thursday to try to push forward talks to sign a crucial bilateral security deal and halt the release of prisoners the United States considers a threat to security.
Afghanistan plans to release hundreds of prisoners from Bagram prison, which was handed over from U.S. control only after a deal was reached in March after intense negotiations because Washington feared dangerous inmates would be freed.
The disagreement further strains relations between the two countries, which are already at breaking point over President Hamid Karzai's refusal to sign a bilateral security deal to shape the post-2014 U.S. military presence in the country.
Without the pact, Washington could pull most of its troops out after this year, when most foreign troops are due to exit.
"If these releases go ahead, it will do irreparable damage to the relationship," said Graham, a Republican from South Carolina, at a news conference in Kabul.
Washington considers 88 of some 650 prisoners marked for release a serious threat to security, saying they are responsible for wounding or killing 57 Afghans and 60 U.S. and coalition troops.
It wants them to be investigated and tried. Afghanistan says there is not enough evidence to keep them detained.
"We're going to have to just wait and see what happens ... we can't go any further in our comments except that it does damage," said McCain, R-Ariz., a member of the Senate Foreign Relations Committee.
"As a result of our long meeting with president Karzai we have narrowed those differences and I believe we can look forward to signing the Bilateral Security Agreement ... sooner rather than later."