The U.S.-led International Security Assistance Force said the cause of the crash is under investigation but initial reporting indicates there was no enemy activity in the area at the time.
It did not immediately identify the nationalities of those killed. But a senior U.S. official confirmed they were Americans. The official spoke on condition of anonymity because he wasn't authorized to release the information ahead of a formal announcement.
The deaths raised to nine the number of Americans, including three civilians, killed in Afghanistan so far this month.
A local official, Mir Baz Khan, said the helicopter crashed in an agricultural field in the Pachir Wagam district in Nangarhar province.
Shir Azam, a teacher who lives in a village near the site, said he heard a loud explosion, then saw the helicopter in flames as it plunged to the ground.
Then, he said, more helicopters came and American troops sealed off the site. He also said he heard nothing to indicate any shooting before the crash.