U.S. efforts to persuade Afghan President Hamid Karzai to sign a long-term security agreement according to Washington's timetable will likely fail, the lead American negotiator has warned the Obama administration, Reuters reported.

In a classified cable that the Washington Post said was transmitted in recent days, U.S. Ambassador James Cunningham wrote that he did not think Karzai would agree to sign the agreement before Afghanistan's presidential election in April, the newspaper said, citing U.S. officials who spoke on condition of anonymity.

The United States wants the Afghanistan government to sign the agreement in matter of weeks if a contingent of U.S. troops is to remain there after 2014, the White House said.

Without a deal, the United States could pull out all troops, the "zero option," leaving Afghan forces to battle the Taliban on their own.

Karzai has called that an empty threat and suggested any security deal could wait until after the April elections.

The United States has 46,000 troops in Afghanistan, but that figure is set to fall to 34,000 by early 2014.

In a another move likely to strain U.S.-Afghan ties, a spokesman for Karzai said on Thursday that Afghanistan had enough evidence to try only 16 of 88 prisoners the United States considers a threat to security and plans to free the remaining detainees.

Washington strongly opposes their release because it says the prisoners being held in Afghanistan have been involved in the wounding or killing of U.S. and coalition troops.