National Security Adviser Susan Rice on Monday warned Afghan President Hamid Karzai that his refusal to sign a post-war security accord was “not viable” and that further delays would force the U.S. to begin planning for the withdrawal of all forces from Afghanistan.

Rice met with Karzai at the end of a three-day trip to Afghanistan, amid mounting tensions between Washington and Kabul.

“Ambassador Rice conveyed to President Karzai that the United States welcomes the Loya Jirga's overwhelming endorsement of the U.S.-Afghanistan Bilateral Security Agreement (BSA) and is prepared to sign the agreement in the coming days,” said the White House in a statement.

But Rice’s diplomatic efforts appear to have fallen short. The White House said Karzai “outlined new conditions for signing the agreement and indicated he is not prepared to sign the BSA promptly.”

“Ambassador Rice stressed that we have concluded negotiations and that deferring the signature of the agreement until after next year's elections is not viable, as it would not provide the United States and NATO allies the clarity necessary to plan for a potential post-2014 military presence,” the White House added.

“Ambassador Rice reiterated that, without a prompt signature, the U.S. would have no choice but to initiate planning for a post-2014 future in which there would be no U.S. or NATO troop presence in Afghanistan.”

The U.S. and Afghanistan agreed to a security deal governing the role of American forces post-2014, after the end of the NATO mission in the country. The Obama administration hopes to leave a small force to continue training Afghan national security forces and prevent a Taliban resurgence.

Karzai though has said he will not any security deal with the U.S., despite its approval by the Loya Jirga — an influential council of Afghan elders. Karzai, citing “mistrust” between him and Washington, said last week that he would let his successor sign the deal. The Afghan presidential elections are slated for 2014, meaning an agreement would not be signed until the following year.

The administration has urged Karzai to quickly sign the deal by year’s end, saying that delaying would hamper plans to transfer additional foreign aid and humanitarian assistance to Afghanistan and plan for the post-war role of U.S. forces.

White House press secretary Jay Carney said last week that the U.S. had presented its “final offer” to Afghanistan and was not open to renegotiating the terms.