Afghan President Hamid Karzai must sign a bilateral security deal on the future of American troops in the war-torn country as soon as possible, a senior US official insisted Monday.

"We've said that this needs to be signed as soon as possible," State Department deputy spokeswoman Marie Harf said, referring to the pact to govern the presence of US troops in Afghanistan beyond late 2014 when international forces will be withdrawn.

"Our position on that hasn't changed, and we'll continue the discussions with President Karzai, with the Afghans about how to get this done as soon as possible," Harf added.

US Secretary of State John Kerry in an interview with ABC television aired on Sunday appeared to open the door to allowing Karzai's successor to sign the bilateral security deal, saying: "I believe that Hamid Karzai, either he or his successor, will sign this."

But presidential elections in Afghanistan are not due until April, and Kerry also stressed that Karzai "needs to sign this."

The deal was hammered out during tough negotiations, which said it should be agreed by the end of the year. But after all the terms were agreed, Karzai backtracked and said it was up to the next president to sign it.

His move has infuriated Washington which says without a deal in place it will be impossible to plan for a post-2014 presence set to help train Afghan security forces.

However with just over two weeks of the year left, the reality is that the deal is unlikely to be signed by December 31, and Harf stressed: "I don't want to put hard and fast deadlines on things."