CCTV footage has emerged of a knife attack on one of the Americans who overpowered a gunman on a Paris-bound train.

Spencer Stone suffered what police described as "significant" injuries during a street brawl while out with friends in Sacramento, California.

The US airman is in a stable condition in hospital after reportedly being stabbed four times in the chest.

It is not clear whether the CCTV footage shows the moment Mr Stone was stabbed.

The incident, which took place at 12.45am (local time) on Thursday morning, is not thought to be connected to terrorism.

Sacramento Police Deputy Chief Ken Bernard says he wants to make two things clear.

Some reports suggest Mr Stone was protecting a female friend.

Officers sealed off a two-block area after the stabbing and interviewed a man and a woman - but no arrests have been made.

Mr Bernard said his team were hunting two Asian men in their 20s who fled in a dark Toyota Camry.

The injuries were so severe that it was initially thought Mr Stone would not survive and a homicide investigation was launched, according to KCRA-TV.

Mr Stone - along with friends Alek Skarlatos and Anthony Sadler - was hailed as a hero in August after subduing a heavily armed gunman on a train heading from Amsterdam to Paris.

During the scuffle, Mr Stone was stabbed with a bolt cutter in the neck and was sliced in the hand by the attacker.

He recalled at the time: "He seemed like he was ready to fight to the end. So were we".

Mr Stone, who was also praised for helping to stop the bleeding of a wounded French-American passenger, said his thumb had to be reattached following the incident.

The three heroes, along with Briton Chris Norman, were awarded France's highest honour, the Legion d'Honneur, for their bravery.

Following Thursday's stabbing, Mr Skarlatos tweeted: "Everybody send prayers out to the stone family today".

The attack on his friend was Mr Skarlatos' second brush with bad news in a week. The Oregon National Guardsman attends Umpqua Community College, where a gunman killed nine people last Thursday.

The 22-year-old would have been in classes when the shooting happen had he not been competing in the US television show Dancing With The Stars.

Watch the footage below: