The senior U.S. commander in Afghanistan says if the American troop level is cut to 5,500 by year's end as President Barack Obama has proposed, there will be too few to train the still-fledgling Afghan security forces.

Army Gen. John F. Campbell's assessment underscores the risks of Obama's goal of ending the war before he leaves office in January 2017.

To leave the Afghans without American military trainers could imperil the gains made since 2002, when the U.S. began committing billions of dollars to rebuild the country. Nearly $64 billion has been allotted for building up the Afghan army and police.

Yet patience in Congress is fraying with the finish line so far away. Campbell says the Afghans won't be able to independently sustain their security forces until 2024.