Four were admitted. They are Brigan Forney, Sarah Smith, Matthew Marcinko and Gavin Christian. Brigan Forney is reported to be suffering serious burns.
The crash came in Phitsanulok province during Operation Cobra Gold, the regular military drill which is drawing to a close in Thailand.
Smoke from the heavy US helicopter crash was noticed at noon, soon after a heavy rainstorm. It was 3pm before the injured were ferried out.
Brigan Forney was being treated at Buddhachinaraj Hospital while the other three injured personnel were taken to Pitsanuvej Hospital.
Three other Americans were treated and did not require admission.
About 13,000 soldiers from seven countries - Thailand, US, Singapore, Indonesia, Japan, South Korea and Malaysia - have been participating in Cobra Gold 2013, a joint, multi-national military training exercise.
It's believed the exercise was drawing to a close.
Four Thai military personnel were slightly injured when a helicopter staged an emergency landing on Phuket on February 6.