A CH-46 Sea Knight helicopter operated by US Marines taking part in the Cobra Gold exercise made an emergency landing near Phu Hin Rong Kla mountain in Phitsanulok province, Thai air force chief Prajin Jantong said.
ACM Prajin said initial reports indicated one of the seven or eight people aboard the helicopter was injured, but the others were safe.
Two helicopters, one from the US and one from the Thai army, were despatched to the area to check.
The air force chief said he had instructed the chiefs of the Directorate of Operation and air safety office to put personnel on alert to provide assistance if requested.
Further details were not yet available, ACM Prajin said.
Earlier reports said a US aircraft crashed near Ban Khek Noi in Nakhon Thai district of Phitsanulok province about 11.30pm and all crewmen were feared dead.