WASHINGTON (AFP) - The White House insisted on Friday it is "imperative" that the United States and Afghanistan sign a new security pact this year, after the Afghan president said it could wait until his successor is elected.

"We need it done by the end of the year," spokesman Jay Carney told reporters, as Washington wrangled with President Hamid Karzai over the rules to govern the presence of US troops in Afghanistan after 2014.