Uncertainty over security clouds NATO talks on Afghanistan --- 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. -- 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. - More