With peace talks stalled, violence in Afghanistan has escalated spiked sharply as the insurgent group makes rapid territorial gains. A US defense official said on Wednesday that Taliban fighters could take over Kabul in 90 days.
Khan said a political settlement was looking difficult under current conditions.
"I tried to persuade the Taliban... three to four months back when the Taliban senior leadership came here," Khan told foreign journalists at his home in Islamabad.
"The condition is that as long as Ashraf Ghani is there, we (Taliban) are not going to talk to the Afghan government," Khan said, quoting the Taliban leaders as telling him.
Peace talks between the Taliban, who view Ghani and his government as puppets of the United States, and a team of Kabul-nominated Afghan negotiators started last September but have made no substantive progress.