NATO opened negotiations with Afghanistan on Saturday over a proposed military force to stay in the country after next year, but said no deal would be signed until after a separate agreement with the US was completed.

Afghan President Hamid Karzai and the US have been in a long and bitter standoff over the Bilateral Security Agreement, 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.

"The message of the United States and its allies in Europe is clear: the Bilateral Security Agreement should be signed without any more delay," Pentagon spokesman Rear Admiral John Kirby said in a statement.

NATO Secretary General Anders Fogh Rasmussen said negotiations had begun between NATO senior civilian representative Maurits Jochems and Afghan national security adviser Rangin Spanta.

"I welcome the start of these talks today... while stressing that the NATO Status of Forces Agreement will not be concluded or signed until the signature of the Bilateral Security Agreement between the Governments of Afghanistan and the United States," Rasmussen said in a statement.

Hagel hailed the start of negotiations and was "pleased" by the move, according to his spokesman.

"NATO's decision to move forward with negotiations on a SOFA is yet another demonstration of the international community's willingness to support Afghanistan after 2014."

Rasmussen 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."

A complete pullout of foreign troops would raise fears of a collapse of the still fragile Afghan forces and a resurgence of the extremist Taliban regime that was ousted in a US-led invasion in 2001.