Three Americans are being hailed as heroes for tackling and disarming a gunman on board a high-speed train between Amsterdam and Paris.

US Air Force serviceman Spencer Stone remains in hospital after being stabbed as he confronted the man on the high-speed Thalys train, which was travelling through Belgium. His injury is not thought to be life-threatening.

Alek Skarlatos, a National Guardsman from Roseburg, Oregon, and Sacramento State University student Anthony Sadler also helped subdue the gunman.

Anthony Sadler, a senior at Sacramento State University, and Alek Skarlatos, US National Guardsman from Roseburg, Oregon, who helped overpower the high-speed train attacker. Photo: Facebook/AP

Another passenger was wounded by a handgun in the attack and also remains in hospital, according to the mayor of the northern French city of Arras, where the train eventually stopped.

It is unclear whether there was a political motive to the gunman’s actions. French authorities are questioning the attacker, identified by police as a 26-year-old of Moroccan origin, and are expected to speak to at least one of the Americans shortly about what happened.

Counter-terrorism police are leading the investigation, according to the Paris prosecutor’s office.

French Interior Minister Bernard Cazeneuve, speaking in Arras, said the Americans “were particularly courageous and showed great bravery in very difficult circumstances,” and that “without their sang froid we could have been confronted with a terrible drama”.

He called for caution before jumping to conclusions.

French authorities are on heightened alert after Islamic extremist attacks in January left 20 people dead, including the three gunmen. In June, a lone attacker claiming allegiance to Islamic radicals beheaded his employer and set off an explosion at an American-owned factory in France, raising concerns about other scattered, hard-to-predict attacks.

Anthony Sadler was travelling with childhood friends Mr Stone, of Carmichael, California, and Mr Skarlatos, when they heard a gunshot and breaking glass.

Mr Sadler told reporters they saw a train employee sprint down the aisle followed by a gunman with an automatic rifle.

“As he was cocking it to shoot it, Alek just yells, ‘Spencer, go!’ And Spencer runs down the aisle,” Mr Sadler said.

“Spencer makes first contact, he tackles the guy, Alek wrestles the gun away from him, and the gunman pulls out a boxcutter and slices Spencer a few times. And the three of us beat him until he was unconscious.”

The Americans ‘were particularly courageous and showed great bravery in very difficult circumstances’

Another passenger helped tie the gunman up, and Stone then quickly turned to help another passenger who had been wounded in the throat, stopping his bleeding until paramedics came, Mr Sadler said.

A British passenger told reporters in Arras that he also helped subdue the suspect.

The identity of the person with the gun wound has not been released, and it is unclear whether the victim was intentionally targeted.

Mr Sadler said: “The gunman never said a word.”

Forensic officers are currently carrying out inquiries in and around the cordoned-off train in Arras.

US President Barack Obama said: “While the investigation into the attack is in its early stages, it is clear that their heroic actions may have prevented a far worse tragedy.”

Stone will undergo surgery but he is doing “relatively well,” Arras mayor Frederic Leturque said.

Mr Skarlatos, 22, had returned from a deployment in Afghanistan in July, and Stone is stationed in the Azores, according to his stepmother Karen Skarlatos.

She spoke with her step-son immediately after the incident. “He sounded fine, but he was intense - he sounded like he had just thwarted a terrorist attack.”

The Arras mayor praised the “extraordinary reflexes” of the Americans and awarded them special medals overnight.

“I wanted them to feel recognition not only from the city but also from French people in general and from all people who are against terrorism,” he said.

“We avoided the worst, but the situation was tough, for them and for everyone,” he said.

The attacker did not fire his automatic weapon but wounded one man with a handgun and the other with a blade, said Philippe Lorthiois, an official with the Alliance police union.

A third person, French actor Jean-Hugues Anglade, suffered a minor injury while activating the train’s emergency alarm, Mr Lorthiois said.

Europe’s major rail stations, such as Paris’ Gare du Nord and Brussels’ Gare du Midi, are being patrolled by soldiers armed with rifles, but passengers can board most high-speed trains without passing through metal detectors or having their bags searched or showing their passports.

A Spanish official said the suspect had previously travelled to Syria.

The official, linked to Spain’s anti-terrorism unit, said the individual lived in Spain until 2014, then moved to France before travelling to Syria, and then returning to France.

Meanwhile, Mr Cazeneuve said that the suspect may be a 26-year-old Moroccan flagged by Spanish authorities last year for links to Islamic radical movements. However, he added that the suspect’s identity has not been 100 per cent confirmed.