Stoltenberg told reporters at a press conference in Brussels that the alliance will increase the number of troops in Afghanistan from 13,000 to 16,000 to train Afghan Special Operations Forces as part of its Resolute Support operation.
"We have decided to increase the number of troops ... to help the Afghans break the stalemate, to send a message to the Taliban, to the insurgents that they will not win on the battleground," he said.
The troops will not perform combat operations, but will focus on training, assisting and advising Afghan troops.
"We are going to help them with developing their air force," Stoltenberg said. "The Afghans are now more and more capable of conducting air operations themselves, and we will help them with military schools, improved command and control."
Stoltenberg added about half the new troops would come from the United States, while the rest would be supplied by the other 28 NATO member nations.
"Based on what we have seen so far indications are roughly half U.S., half non-U.S. of the additional forces, which is also actually the case now for the 13,000 troops we have there today," he said.
Defense Secretary James Mattis said in September he planned to sign an order sending about 3,000 additional U.S. troops to Afghanistan to carry out a two-fold mission -- the NATO-led operation to train and assist Afghan forces, and a counterterrorism mission.
In August President Donald Trump said the United States must recommit to the war in Afghanistan, adding "the consequences of a rapid exit are both predictable and unacceptable."