Casey, who serves on the powerful Senate Foreign Affairs committee, also noted that the U.S. has made significant military advances against the Taliban during the last year.

"The dynamic has changed," said Casey. "Last year, you could make the case that the Taliban had the upper hand, but now you can make the case that they are back on their heels. It's at worst a stalemate. The military has made great strides, but it's fragile."

The U.S. military is currently scheduled to begin withdrawing troops from the country next spring while turning over power to the Afghan national army.