NATO would have to pull all its troops out of Afghanistan by the end of 2014 if Afghan President Hamid Karzai does not sign a security pact with the United States, alliance chief Anders Fogh Rasmussen has said.
The statement came on Monday, weeks after an assembly of Afghan elders, the Loya Jirga, endorsed the security pact intended to shape the US military presence in the country beyond 2014.
Karzai has said he might not sign it until after elections in April.
The NATO-led force currently has around 80,000 troops in Afghanistan, the majority American. NATO is winding down combat operations, handing responsibility for fighting Taliban fighters to the Afghans, before most foreign combat forces pull out by the end of 2014.
NATO plans to leave a training mission, expected to number 8,000 to 12,000 soldiers, in Afghanistan after 2014.
The US has already warned it could withdraw all its forces by the end of next year, the so-called "zero option", if Karzai does not sign the pact.
Without the US-Afghan accord, NATO will not be able to finalise its own agreement with the Afghan government setting the terms for troops from other NATO and partner nations to remain in Afghanistan after 2014, Rasmussen told reporters.
"In that case, we don't have a proper legal framework in place and it will not be possible to deploy a 'train, advise, assist' mission to Afghanistan after 2014," Rasmussen said.
He voiced hope Karzai would follow the advice of the Loya Jirga and sign.
The agreement that NATO needs with Afghanistan is modelled on the proposed US pact and, in any case, Washington is expected to supply most of the forces for the post-2014 NATO mission, so without the US, the mission is unlikely to be feasible.
Although terms of the Afghan-US pact were settled after a year of wrangling, Karzai has since added conditions including the release of all Afghan prisoners from Guantanamo Bay in Cuba and an end to military operations involving Afghan homes.