Afghan President Hamid Karzai and the US have been in a long and bitter stand-off over the bilateral security agreement (BSA), which would see several thousand US troops deployed in Afghanistan after 2014.
Karzai initially endorsed the BSA, but has since declined to sign it, outraging US officials and lawmakers who have threatened a complete forces pullout.
NATO Secretary General Anders Fogh Rasmussen said negotiations had begun between NATO senior civilian representative Maurits Jochems and Afghan national security adviser Rangin Spanta.
He said the Status of Forces Agreement was essential for the NATO-led mission to "mission to train, advise and assist" the Afghan army and police after 2014.
Karzai, who is due to stand down after elections next year, has warned that he will not allow a continued NATO presence if it meant "more bombs and killings".