In more than seven years of fighting, insurgents have only rarely managed to down Western helicopters. Choppers are a crucial mode of transport for troops and supplies, because many of Afghanistan's roads are poorly maintained and dangerous, and Western bases are widely scattered amid extremely rough terrain.
Also Monday, a suicide bomber struck American police trainers who were meeting with Afghan police officials, Pakistan's Dawn newspaper reports.
The bomber entered the police station while Afghan officials were meeting US troops advising a police training programme, provincial police chief Gen Abdul Rahman Sayed Kheil said. The blast killed two American soldiers who were beside a Humvee, news footage of the scene showed.
US forces in Afghanistan confirmed that two "service members" from the US-led coalition were killed and three were wounded.
A Taliban spokesman, Zabiullah Mujahid, claimed responsibility for the blast.
More U.S. and NATO troops have died this year in Afghanistan than any other year since the 2001 U.S. invasion, in part because Taliban militants are launching increasingly complex and deadly attacks.