Hopes for ending America's longest war hinge on maintaining a weeklong fragile truce in Afghanistan that U.S. officials and experts agree will be difficult to assess and fraught with pitfalls.

The seven-day "reduction of violence" deal promised by the Taliban will begin on Friday night, a senior U.S. State Department official said, without specifying the exact time. That will start the countdown to the signing of a peace agreement between the Taliban and the United States at the end of the month.