KABUL (Reuters) - The United States is preparing to increase the number of troops it keeps in Afghanistan in 2015 to fill a gap left in the NATO mission by other contributing nations, according to three sources with direct knowledge of the situation.

The final numbers are still being agreed, but there will be at least several hundred more than initially planned, one of the sources said. "If they hadn't done that, the mission would have lost bases," the source said.

Under the US commitment, described as a "bridging solution" until other nations fulfil their pledges later in the year or the troops are no longer needed, Washington may provide up to 1,000 extra troops.

That figure was confirmed by all three sources, who said the final number was still under discussion and depended on when other countries stepped forward with their commitments.