NEW : Battle lasted 12 hours as two of militants fired on Afghanistan , official says . Eight U.S. troops , hundreds members of Afghan National Security Force killed in fighting . Coalition forces say they called in airstrikes , repelled attack from militants . Incident marks deadliest day for U.S. troops in outpost since more than a year .
NEW : Battle lasted 12 hours as hundreds of militants fired on outpost , official says . Eight U.S. troops , two members of Afghan National Security Force killed in fighting . Coalition forces say they called in airstrikes , repelled attack from militants . Incident marks deadliest day for U.S. troops in Afghanistan since 2008 .