Brussels (CNN) -- NATO warned it may be forced to withdraw all of its troops from Afghanistan by the end of next year if President Hamid Karzai doesn't sign a security agreement with the United States.
Reached last month, the pact lays out the U.S. military presence in Afghanistan after 2014, when the NATO-led force of some 80,000 troops is scheduled to leave.
Although the agreement was endorsed by the Afghan council of tribal leaders, called the Loya Jirga, Karzai said he won't sign it until after elections in April.
Uncertainty over the agreement is causing mounting anxiety among diplomats here, where Afghanistan is dominating talks among NATO ministers, including Secretary of State John Kerry.
"This is not fooling around. This is serious business. There are over 50 nations who are engaged here through NATO in trying to help Afghanistan," Kerry said in Brussels.
"Can you maybe muddle through? Can you do other things? That is not the issue here," Kerry added. "The issue is how do you get the best transition possible."
NATO wants the agreement with the United States signed before it finalizes its own accord with the Afghan government, as it is to be modeled after the one Afghanistan reached with the Obama administration.
Without it, Secretary General Anders Fogh Rasmussen warned the alliance would be unable to shape a future military presence.
"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," he said, adding he hoped Karzai would follow the advice of the Loya Jirga and sign the agreement.
"It is my firm hope and intention, therefore, to continue our efforts to support Afghanistan, once these agreements are concluded," Rasmussen said.
Because the United States is expected to supply the bulk of forces for any post-2014 mission, any presence is unlikely without a signed agreement between Washington and Kabul.
Karzai has said he won't sign until certain conditions are met. These include promises from the United States of no more raids on Afghan homes and that Afghan prisoners will be released from the Guantanamo Bay detention center.
Washington has resisted such changes, saying Karzai's demands are new provisions added to a deal that was already reached. Kerry noted that President Barack Obama has urged Karzai to sign before the end of the year.
National Security Adviser Susan Rice traveled to Kabul last week, where, according to senior U.S. officials, she warned Karzai the United States would begin pulling out troops in 2014 if he failed to sign the pact by the year-end timeframe.
"We consider that the Loya Jirga represents the will of the Afghan people, and it's time to move on with it," a senior State Department official said.
Washington fears the stalemate could encourage its war-weary partners to abandon plans to keep troops beyond 2014. Senior U.S. and NATO officials said several countries have voiced concern about the delay.
A withdrawal of NATO troops would also affect international aid totaling $8 billion a year, half of which is needed to finance Afghan security forces.
On Wednesday, ministers from 28 NATO allies and 41 nations will meet with acting Afghan Foreign Minister Zarar Ahmad and Interior Minister Mohammad Daudzai, where they are expected to warn the Afghans about the alliance's "zero option" concern.
"It's not the choice of any of our NATO allies or partners to go there. We all want to be able to continue to train, assist, advise the Afghans," a senior State Department official traveling with Kerry told reporters. "It's not just the United States that the Afghans are going to hear from. They're going to hear from 60-plus countries."