LOS ANGELES, California (AFP/REUTERS) - One of three Americans who helped thwart a terror attack on a train in France, was recovering on Friday after being stabbed in his home state of California, hospital officials said.

Spencer Stone "is awake, able to get out of bed and in good spirits," a statement by the UC Davis Medical Centre said.

The hospital also upgraded the 23-year-old airman's condition from serious to fair.

Stone underwent two hours of surgery and is expected to recover, according to the hospital.

A witness who saw the stabbing, meanwhile, said Stone was trying to stop a fight between a man and a woman outside a bar Sacramento, California, when the incident occurred.

Eric Cain, who works at a liquor store near the bar, told the CBS Sacramento affiliate KOVR on Thursday that a woman was arguing outside the bar with a man who appeared to be her boyfriend.

She hit him with a plastic bag, and he then punched her in the face, Cain said.

"The big dude, the white guy, stood up and kinda got in his face," Cain said, referring to Stone.

In the fight that ensued early on Thursday, Stone was stabbed several times in his torso, authorities said.

"I saw the back of his shirt. I saw a big red mark on the back of his shirt, and another random person walked by, and I just kinda went, 'I think the dude got stabbed,'" he said.

Stone and two of his friends helped foil an attack in August on a Paris-bound train. They have since been hailed as heroes both in the United States and in France, where they were awarded the Legion of Honour, the country's highest decoration.

Authorities said Thursday's stabbing was not related to Stone's role in the train incident and had no connection to terrorism.

Police have said they are looking for two Asian men suspected in the attack.