Jens Stoltenberg said NATO plans to provide training "outside Afghanistan, focusing on Special Operations Forces", but it is not clear where the training will take place.
The alliance's revised role, after 18 years in the conflict-torn country, would involve "advice and capacity-building support to Afghan security institutions, as well as continued financial support", he said.
"As we end our military presence, we are opening a new chapter."
The US and NATO both began withdrawing troops from Afghanistan on 1 May after US President Joe Biden ordered that all American troops be out of Afghanistan by 11 September.
The US has said it will continue to help Afghan forces and monitor the threat that prompted it to invade the country after the 11 September 9/11 terror attacks in New York and Washington in 2001.
America's allies in Europe, as well as Canada, rely on US logistics and transport to operate in Afghanistan, with their troops obliged to leave the country as well.
Many officials have expressed concern that once the US leaves, the Afghan government and its military will be quickly overrun by the Taliban.
It remains unclear what level of security might be needed to protect international embassies in Kabul, the Afghan capital, and who would provide this.
The city's airport will also need protection.
Mr Stoltenberg said NATO plans to "fund the provision of services, including support for the functioning of Kabul airport".
Earlier this month, the US military officially handed over a facility to the Afghan National Army as part of its troop drawdown from the country.
It comes as violence surges in Afghanistan, with roadside bombings in southern and central areas killing 13 people on Thursday, officials said.
Meanwhile, militants stopped a bus in the west of the country, ordering three men to get out before shooting and killing them.
No one claimed responsibility for the attacks, while the government blamed the Taliban, which denied responsibility.
America's longest-serving commander in Afghanistan told the BBC earlier this week that "history will judge" the success of coalition forces' presence in the country.
Cover image: NATO Secretary General Jens Stoltenberg (Picture: NATO TV).