"We have just endorsed the detailed concept of our non-combat mission in Afghanistan" to guide military planners as they prepare NATO's advice and training programme, alliance head Anders Fogh Rasmussen said.
Rasmussen said "Resolute Support," will be based on a "limited regional approach", with operational centres in Kabul and around the country to train and advise some 350,000 Afghan government troops.
Asked about staffing levels for the new mission, Rasmussen said: "We have not decided the exact number."
It has not yet been decided how many will remain, with initial estimates of 8,000 to 12,000 now seen as possibly too low in the face of continued Taliban attacks which have stoked concerns the Afghan government remains vulnerable.
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On Monday, the Pentagon said it was considering plans for a larger, temporary US force to remain behind for up to three years to provide air power, medical evacuations and other support to the Afghan forces.
"We intend to be there for the long haul... We have time to sort out specific numbers," Hagel said. "I think we are all on the same page on this."
Afghan government forces are plagued by a myriad of problems, including a high rate of desertion, weak logistics and a fledgling fleet of helicopters.
Rasmussen also stressed the international community's commitment but said that "ultimately it is for the Afghans to determine their own future".
US-led forces invaded Aghanistan to oust the Taliban after the 2001 terror attacks on the US.