August has been the deadliest month for U.S. forces in Afghanistan since the conflict began nearly 10 years ago.
Sixty-six American troops have died this month, topping July 2010 when 65 troops died, according to a CNN tally.
Almost half the August troop deaths took place on August 6 when insurgents shot down their helicopter in the eastern central province of Wardak. The Taliban claimed militants downed the helicopter with a rocket-propelled grenade.
Thirty U.S. service members - including 17 Navy SEALs - were killed in that attack, the single largest loss of life for U.S. troops since the Afghan war began in late 2001.
In contrast, 36 U.S. service members were killed in all of July. Prior to the August attack, the most U.S. troops killed in a single month this year was 47 in June.
A "surge" of 33,000 additional troops in 2009 - in response to increased insurgent attacks - led to an uptick in U.S. deaths over previous years, with 499 killed in 2010. Prior to the surge, the most U.S. troops killed in a single year was 155 in 2008, according to CNN figures.
The surge in U.S. deaths comes as NATO is drawing down and handing over security control to national forces. Some 10,000 U.S. troops are scheduled to depart by year's end, with all U.S. military personnel out of Afghanistan by the end of 2014.