U.S. and NATO officials are applying new pressure on Afghanistan's leaders to allow thousands of foreign troops to remain in the war-torn nation beyond next year.

They want some troops to stay in Afghanistan to continue training and advising local forces after 12 years of war - and the departure of international combat forces by the end of 2014. Afghan President Hamid Karzai has resisted signing a new security agreement with the U.S.

U.S. Secretary of State John Kerry said Tuesday at NATO headquarters in Brussels that there is more at stake than bilateral relations between Afghanistan and the United States.

"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, and those nations have budget cycles. Those nations have planning requirements. Those nations have equipment requirements. They have deployment requirements, and all of those things are best managed with planning."

Kerry said Afghanistan's vast majority, represented by the council of tribal elders known as the Loya Jirga, has made a judgement to sign the security agreement in order to optimize the security transition in Afghanistan.

The U.S. has urged Afghan leaders for a final decision by the end of this month.

NATO chief Anders Fogh Rasmussen said Tuesday financial aid for Afghan security is at risk if NATO is not allowed to deploy its own trainers to Afghanistan after 2014. He also said international aid for Afghan development might also be put at risk if Afghanistan does not sign the security agreement.

Kerry is taking part in a two-day meeting of NATO foreign ministers in Brussels.