The United States may have to deploy hundreds of additional forces in coming months, beyond the 9,800-strong contingent announced previously, because of a shortfall in troop contributions from NATO members, defense officials told AFP.

Commanders were still working out the details but there was a shortage of roughly 400 to 700 NATO troops that would need to be filled temporarily by US forces through the winter months of 2015, said a defense official who spoke on condition of anonymity.

"This is just to bridge the gap," the official said.

The shortfall was a result of the delayed signing of a bilateral security agreement this year between Washington and Kabul, which also complicated deals with other NATO countries to deploy troops to Afghanistan starting next year.

The post-2014 NATO mission calls for 12,500 troops to remain on the ground starting in January, with about 2,700 non-US forces.

Military officers had indicated earlier this month to AFP that the postponed signing of the US-Afghan security agreement due to a protracted electoral dispute had raised doubts about whether allied governments could have arrangements in place for troops to deploy by 2015.