Spencer Stone, one of the three Americans who helped thwart a terror attack on a French train, has been "repeatedly stabbed".

Police said Stone was stabbed in Sacramento around midnight on Wednesday night, NBC reports.

He was said to be in a stable condition.

In a statement, the Air Force said he was "alive and in stable condition".

"We do not have any information as to the events preceding the incident," the statement added.

Airman First Class Stone, 25, was one of the men who tackled Moroccan national Ayooub El-Khazaani after he opened fire with an AK-47 on a high-speed train from Amsterdam to Paris in August.

Stone tackled Khazaani, along with two other American men, and put him in a chokehold which led to the terror suspect passing out. He was stabbed with a box cutter in the process and needed surgery to his hand.