The United States "will not hesitate to nullify" its historic deal with the Taliban if the insurgents renege on their security guarantees and commitment to hold talks with the Afghan government, US Defense Secretary Mark Esper said on Saturday.

His comments came as the US signed an accord with the Taliban in Doha, laying out a timetable for a full troop withdrawal from Afghanistan within 14 months as it seeks an exit from its longest-ever war.