Off-duty members of the U.S. military subdued a gunman "known to intelligence services" after he opened fire, injuring three aboard on a high-speed train en route to Paris from Amsterdam Friday.

Three U.S. servicemen were on board the train and overpowered the man when the train stopped in the northern French city of Arras, 115 miles north of Paris, French media reported. Some reports said the men were U.S. Marines but that could not be confirmed.

Passengers on the train subdued the gunman and prevented further carnage, said Christophe Piednoel, spokesman for national railway operator SNCF. The train was then diverted to Arras, where police arrested the suspect, Piednoel said on French television i-Tele.

The suspect was arrested after the train stopped in Arras, 115 miles north of Paris, Interior Ministry spokesman Pierre-Henri Brandet said on French television BFM. Passengers were evacuated and police have secured the area.

The man was armed with an automatic rifle and a knife, Piednoel said.

The suspect is a 26-year-old Moroccan, Alliance police union official Sliman Hamzi said on French television i-Tele.

The victims were identified as an American, a Briton and a Frenchman. Earlier reports said two American service members were among the injured. Two of the victims were in critical condition, according to a statement from the office of President Francois Hollande.

"The situation is under control, the travelers are safe. The train stopped and the emergency services are on site," Thalys, the train operator, tweeted.

Aug. 21: A Thalys train of French national railway operator, SNCF, stands at the main train station in Arras, northern France, after a gunman opened fire injuring three people. (AP)

The attack took place while the train was passing through Belgium, according to a statement from Hollande's office. The statement said he spoke with Belgian Prime Minister Charles Michel, and the two leaders pledged to cooperate closely on the investigation.

Two of the victims were considered to be seriously injured, the French state rail company SNCF said, according to French wire service AFP. Interior Minister Bernard Cazeneuve is on his way to the scene. Passengers were evacuated and police have secured the area.

The motivation for the attack is unknown, officials said. AFP cited French officials saying the suspect is "known to intelligence services."

Investigators from France's special anti-terror police are leading the investigation, a spokeswoman for the Paris prosecutor's office said.

"As always where an act that could be terrorist in nature is involved, the greatest care and the greatest precision will be used," Cazeneuve said.

Cazeneuve said the two Americans "were particularly courageous and showed great bravery in very difficult circumstances" and that "without their sangfroid we could have been confronted with a terrible drama."

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

Passenger Christina Cathleen Coons of New York described the drama in car 12 of the train in an interview with Ouest France newspaper.

"I heard shots, most likely two, and a guy collapsed," she is quoted as saying.

Coons, identified as a 28-year-old vacationing in Europe, said a window broke above one woman's head. "A guy fell to the floor and had blood everywhere," she is quoted as saying.

She described lying on the floor herself and taking photos with her phone.

"I thought there would be a shootout in the train," the newspaper quotes her as saying. Then, "people came to take care of him."

A White House official told Fox News that President Obama was briefed on the incident Friday evening:

"The President's thoughts and prayers are with all of the victims of this attack, and he wishes them speedy and full recoveries," the official said.

"Echoing the statements of French authorities, the President expressed his profound gratitude for the courage and quick thinking of several passengers, including U.S. service members, who selflessly subdued the attacker. 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. We will remain in close contact with French authorities as the investigation proceeds."

The Associated Press contributed to this report.