Mattis' remarks came as he flew into the war-torn city, weeks after Afghan President Ashraf Ghani unveiled a plan to open peace talks with the Taliban. The insurgents have so far given no formal response to Kabul's offer for negotiations.

But Mattis said some insurgent leaders have expressed an interest in discussions.

"It may not be that the whole Taliban comes over in one fell swoop, that would be a bridge too far, but there are elements of the Taliban clearly interested in talking to the Afghan government," Mattis told reporters aboard a military jet.

Ghani's peace plan includes eventually recognising the Taliban as a political party.

The insurgent group has said it was prepared to negotiate, but only with the US and not with the Kabul government.

"Right now we want the Afghans to lead and to provide the substance of the reconciliation effort," Mattis said.