NATO's International Security Assistance Force (ISAF) will finish operations at the end of the year and be replaced by a far smaller training and support mission named "Resolute Support".
But Afghanistan's stability could be at risk without international troops on the battlefield as the country's security forces endure huge casualties in the bloody struggle to thwart the Taliban.
"Next year, we will open a new chapter. The future of Afghanistan will be in Afghan hands, but our support will continue," Secretary General Stoltenberg said on his first visit to Kabul since taking office last month.
ISAF troop numbers peaked at 130,000 in 2010, but now stand at less than 34,000.
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Only about 12,500 soldiers, most of them from the United States, will remain into 2015 to continue training the Afghan forces, who have already taken over most of the fighting.
This year alone more than 4,600 Afghan soldiers and police have been killed in combat, Lieutenant General Joseph Anderson, the number two ranking US officer in Afghanistan, revealed yesterday.
"Those numbers are not sustainable in the long term," he said, adding the Afghan army and police were "holding their ground against the enemy."
President Ashraf Ghani, who came to power in September, said Afghan forces were ready to accept full responsibility, but that NATO's support mission would aid the search for peace after decades of conflict.