It comes amid reports that 1,000 mainly US troops could remain on the ground to protect diplomatic missions and Kabul's international airport.
Nato's 20-year military mission in Afghanistan has all but ended.
But violence in the country continues to rise, with the Taliban taking more territory.
Under a deal with the militant group, the US and its Nato allies agreed to withdraw all troops in return for a commitment by the Taliban not to allow al-Qaeda or any other extremist group to operate in the areas they control.
President Joe Biden set a deadline of 11 September - the 20-year anniversary of the 9/11 attacks on the US - for American troops to fully withdraw, but reports suggest the pullout may be complete within days.
As Afghan forces prepare to take charge of security alone, concern is growing for the future of Kabul.
But speaking to the BBC from the group's office in Qatar, he said no foreign forces - including military contractors - should remain in the city after the withdrawal was complete.