Carney's comments came after Afghan President Hamid Karzai's latest announcement left the outcome of his country's intensely negotiated security deal with the United States in doubt.
Even as Afghanistan's grand council of more than 2,000 elders continued with their four-day Loya Jirga meeting in Kabul on whether to approve the draft of a post-2014 Bilateral Security Agreement on keeping U.S. forces in the country, which had taken months of strenuous negotiations, Afghan President Hamid Karzai sprang a surprise by announcing he won't sign the deal until after the country's presidential election in April, when there will be a new president.
"We have long made clear that we need to get a bilateral security agreement done this year," Carney said. "We've made clear that it's imperative that we do it as soon as possible, and further delay is not practical, nor tenable. ...
"Failure ... would make it impossible for the United States and our allies to plan for a presence post-2014."
The BSA, which must first be approved by the Loya Jirga before it goes for parliamentary and presidential approval, will determine how many American forces would remain in Afghanistan after the U.S. and NATO-led coalition forces end their combat operations of the past 12 years and return home by the end of 2014.
The agreement would also mean billions of dollars of international aid which Afghanistan sorely needs.
Karzai's announcement came as a surprise to all including U.S. officials, who were trying to figure out why Karzai made the statement and what was his intent.
The New York Times reported Karzai also described his relationship with the United States as one of mutual distrust.
The BBC quoted him as saying: "I don't trust them and they don't trust me. The last 10 years has shown this to me. I have had fights with them and they have had propaganda against me."
At her media briefing Thursday after Karzai's statement, U.S. State Department spokeswoman Jen Psaki said, "Well, we're still seeking clarity on the meaning and intention of those specific reported comments."
She reminded that when the United States signed the strategic partnership agreement last year with Afghanistan, both sides "committed to concluding" the BSA within one year.
"We, of course, know the math. That window is this month, and we believe that signing the BSA sooner rather than later is essential to give Afghans certainty about their future before the upcoming elections, and enable the United States and other partners to plan for a U.S. presence after 2014," she said.
Deputy White House press secretary John Earnest also stressed the need for concluding the agreement by the end of this year. He said when Karzai was in the White House early this year there had been "a lot of discussion about the feasibility of a bilateral security agreement, and whether or not that was something that could be agreed to between the two parties."
He said what was important now was that the agreement be "approved and signed by the end of this year" so preparations can start for any post-2014 presence the United States may have in Afghanistan.
"And that's for a very practical reason, which is the presence that's in Afghanistan right now is a NATO presence," he said. "So the United States needs to conduct some planning, both internally but also with our allies, to coordinate what our post-2014 presence would look like."
Asked if Karzai's statement had jeopardized the BSA draft, Earnest would only say, "Well, what we have right now is a text that we've all agreed upon. And what's important is for the Loya Jirga to approve that agreement and for that agreement to be signed before the end of the year. And that is what will allow us to move forward in terms of planning our longer-term relationship."
Psaki in her comment said: "Well, it certainly would need to be agreed to by both sides, of course. And would need to be signed by President Karzai, absolutely."
Other U.S. officials also reminded about the deadline of year-end for the agreement to be signed.
The Washington Post said if Karzai did not sign it by that time, President Barack Obama may be forced to leave no U.S. forces behind after 2014.
Earnest reminded Obama has not decided on the number of residual U.S. troops after 2014, adding: "That's a decision that the president has not yet made. We've not yet determined whether or not a troop presence will continue in Afghanistan."
Any such force would be involved only in military training and counter-terrorism.
The Post quoted administration officials as saying Obama is prepared to decide on and announce the size of the force within weeks, once the document is signed.
Estimates of the residual U.S. forces are that it could be 5,000 to 10,000, while Karzai has said it could go up to 15,000.
The sources, however, told the Post a delay after December would upset U.S. planning and that an April signing as Karzai mentioned would be completely unacceptable.
U.S. Defense Secretary Chuck Hagel was quoted as saying a signed bilateral security agreement would be an assurance that the United States can move forward.
The Post said by describing Karzai's statement as being unclear, the administration may be giving him an opportunity to revise or withdraw his remarks as any scuttling of the agreement would jeopardize the international aid program post 2014.
The New York Times quoted officials as saying Karzai could change his mind if the Loya Jirga approved the deal.
Any force left on Afghan soil would not be involved in "combat operations" except in "mutually agreed" circumstances. The agreement allows an indefinite U.S. presence but is expected to be less than 10 years.
The draft agreement appeared to have resolved two critical issues -- U.S. searches of Afghan homes for terrorist and exempting U.S. forces from Afghan laws for any crimes committed by them.
Obama in a letter to Karzai reportedly said Afghanistan's sovereignty would be respected and U.S. forces would not enter Afghan homes except under "extraordinary circumstances."
Earlier, Amrullah Saleh, former head of Afghan Intelligence, told the BBC he is confident the agreement would be accepted.
"The only thing that gives me high confidence and hope that this will not be a goodbye Jirga, but it will be a Jirga bonding the two nations together is that the stakes are high for the two nations," he said. "Abandoning a country as strategically located as Afghanistan will be committing the mistakes of previous world powers ... the mission is not finished."