WARSAW (AFP) - US Defence Secretary Chuck Hagel on Thursday said the Afghan president's foot-dragging on a crucial bilateral security agreement risked leaving Washington no time to plan its post-combat mission.

"You can't just keep deferring and deferring because at some point the realities of planning and budgeting collide," the Pentagon chief told reporters aboard his plane en route to Warsaw.

NATO combat troops are due to withdraw from Afghanistan by December but the bilateral agreement (BSA) negotiated with Washington allows for a smaller contingent of American troops to remain on the ground.

Without a legal accord however, the United States and NATO would have to drop the idea of a post-2014 force.

Washington originally wanted the BSA signed last October to allow adequate time to plan the NATO training mission which could include up to 10,000 American troops.

But Afghan President Hamid Karzai, whose relations with Washington have steadily deteriorated over the years, refuses to sign.

He has suggested his successor should make the final decision after presidential elections on April 5.

Mr Hagel, who is in the Polish capital to discuss global security issues and the EU member's defence upgrade, said the stalling also poses a problem for fellow NATO members like Poland who may help train the Afghan army after the 2014-pullout.

He added however that Mr Karzai "is the elected president of a sovereign nation, and our ability to influence whatever decisions he makes is limited."