Secretary of State John Kerry expressed confidence Wednesday that the next president of Afghanistan will sign the Bilateral Security Agreement, or BSA, with the U.S., despite a refusal from President Hamid Karzai, who will hand over the country's leadership following elections in April.

Karzai had refused to sign the agreement last year, saying that it is for the next leader to make a decision on such matters. President Barack Obama's administration said earlier this week that it would withdraw its troops from Afghanistan by the end of 2014, leaving behind about 10,000 troops to continue training Afghan forces for counterterrorism operations, Associated Press reported Wednesday. However, if the security agreement is not signed, the U.S. and its allies will not leave any troops behind, leaving the strife-ridden country vulnerable to insurgencies.

"Whether it's signed by him or not we obviously have serious reservations, but all six of the candidates for the presidency of Afghanistan have said that they will sign it," Kerry reportedly said in Washington on Wednesday, according to Agence France-Presse.

A statement from the White House on Tuesday said that the U.S. is looking at a possibility of concluding the BSA with Afghanistan later this year. The statement also said that Obama had told Karzai that because it is unlikely that the latter will sign the agreement, the U.S. will continue planning for a full withdrawal.

“President Obama has asked the Pentagon to ensure that it has adequate plans in place to accomplish an orderly withdrawal by the end of the year should the United States not keep any troops in Afghanistan after 2014,” the statement said.

Army Gen. Martin Dempsey, chairman of the U.S. Joint Chiefs of Staff, expects that the Taliban will become more aggressive this summer and Afghanistan's troops need to be prepared for it.

There are about 19,000 non-U.S. forces currently in Afghanistan, along with 33,600 Americans, according to The Washington Post.  But, without an agreement between the U.S. and Afghanistan, forces belonging to other alliance members will also have to exit Afghanistan, leaving the country exposed to further attacks by insurgents.

NATO Secretary General Anders Fogh Rasmussen said Wednesday about Karzai's reluctance to sign the agreement: “Let me stress, this is not our preferred option. But these are the facts — the facts that we need to take into account in our planning.

“We stand ready to establish the training mission after 2014,” Rasmussen said, adding: “But time is of the essence.”