WASHINGTON - The top U.S. commander in Afghanistan has recommended an increase of 40,000 U.S. troops as the minimum necessary to prevail, two sources familiar with his recommendations said on Thursday.

The sources said General Stanley McChrystal also gave President Barack Obama an option of sending more than 40,000 troops, which could be politically risky given deep doubts among Obama's fellow Democrats about the eight-year-old war.

One of the sources said McChrystal also gave a third high-risk option of sending no more troops.

The sources spoke amid a heated debate in Washington over whether to send more soldiers to Afghanistan to try to put down the Taliban insurgency or to scale back the U.S. mission and focus on striking al Qaeda cells.