* Karzai has said he may not sign security deal until April
* NATO says can't go ahead with post-2014 mission without it
* Billions of dollars in aid at risk -diplomat
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 said on Monday.
An assembly of Afghan elders, the Loya Jirga, last month endorsed the security pact intended to shape the U.S. military presence in the country beyond 2014. But Karzai 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 insurgents 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 United States 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 U.S.-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 U.S. pact and, in any case, Washington is expected to supply most of the forces for the post-2014 NATO mission, so without the United States, the mission is unlikely to be feasible.
Although terms of the Afghan-U.S. 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.
NATO foreign ministers meet in Brussels on Tuesday and Wednesday to discuss Afghanistan. The delay in signing the U.S.-Afghan security pact is causing mounting frustration among NATO diplomats because it is holding up detailed military planning for the post-2014 mission.
NATO officials and diplomats warned privately of dire consequences for Afghanistan, including threats to up to $8 billion a year in aid, if Karzai failed to sign the U.S. pact.
A senior diplomat at NATO said U.S. National Security Adviser Susan Rice had made clear on a recent visit to Kabul that Washington would begin planning to pull out all its troops by the end of 2014 unless Karzai signed by the end of this year.
Foreign donors are required to provide the bulk of the $4 billion a year needed to finance the Afghan security forces.
Withdrawal of all foreign troops could jeopardize that foreign funding as well as around $4 billion a year in civilian aid, the diplomat, speaking on condition of anonymity, said.
"We are concerned, without those eyes and ears on the ground, that the $4 billion a year (in security assistance) will be difficult to appropriate across our capitals because there will simply be less confidence that the money is going to go where it is intended to go," he said.