WASHINGTON: US and Canadian air defenses were placed on heightened alert after a shooting in Canada's parliament, US officials said Wednesday.

The North American Aerospace Defense Command (NORAD) “is taking appropriate and prudent steps to ensure we are adequately postured to respond quickly to any incidents involving aviation in Canada,” said a US defense official, who spoke on condition of anonymity.

The move came as a precaution after a gunman opened fire in and around the Canadian parliament, with a Canadian soldier dying of gunshot wounds and police sealing off the area.

NORAD spokesman Captain Jeff Davis declined to provide details of the steps taken, saying only steps had been taken to ensure defenses were “adequately postured.”

But while the shooting incident in Ottawa raised fears of a potential link to extremists, Davis said there were no signs of possible hijackings or imminent threats to aviation.

“We're not aware of any current, specific threats against the aviation system,” he told AFP.

US intelligence officials were not immediately available to comment as to whether there were any suspected links to extremists in the shooting in Canada.

NORAD, founded during the Cold War, is a combined US, Canadian military command designed to safeguard the air space over the two countries, with its headquarters at Peterson Air Force Base in Colorado.

After the attacks on New York and Washington on September 11, 2001, the command regularly scrambles fighter jets to intercept private aircraft that enter prohibited areas over the US capital or elsewhere, escorting them to the nearest runway.

“Condolences to family of the soldier killed and prayers for the parliamentary guard wounded. Canada will not be terrorized or intimidated,” employment minister Jason Kenney said on Twitter after the shot soldier pronounced dead in Ottawa.

Gunfire echoed through the gothic halls of the Canadian parliament as police swarmed in to tackle the gunman who had shot the soldier outside before storming the building earlier in the day.

One attacker had been shot by police inside the building while up to two more were feared to be at large, as officers sealed off the area.

Video footage posted online by the Globe and Mail newspaper showed police ducking for cover as they advanced along a stone hallway, loud gunfire echoing among the stone columns.

Police officers outside told AFP that one or more – possibly up to three – shooters were feared to be on the roof of the building.

A member of parliament, Maurice Vellacott, told AFP that House of Commons security had told one of his aides that at least one suspect had been killed inside parliament.

Other lawmakers tweeted reports that an assailant had been shot.

“I literally had just taken off my jacket to go into caucus. I hear this 'pop, pop, pop,' possibly 10 shots, don't really know,” Liberal Party member John McKay told CBC television.

“Suddenly the security guards come rushing down the hallways and usher us all out to the back of the parliament buildings.”

CBC radio also reported that one of the suspected shooters had been killed, but there was no official confirmation of this.

Witnesses said they saw a man armed with a rifle running into parliament after the shooting at the memorial.

Passers-by told reporters that a bearded man had gunned down the soldier, hijacked a passing vehicle to take him the short distance to Parliament Hill, on a bluff over the Ottawa River.

One witness, parliamentary aide Marc-Andre Viau, said he saw a man run into a caucus meeting at the parliament, chased by police armed with rifles who yelled “take cover.”

That was followed by “10, 15, maybe 20 shots,” possibly from an automatic weapon, he said. “I'm shaken,” said Viau.

The soldier appeared seriously wounded. Emergency medics were seen pushing on his chest to revive him.

Police raced to seal off the parliament building and the office of Canadian Prime Minister Stephen Harper, pushing reporters and bystanders further back and blocking roads with squad cars.

Harper left the area of the shooting and was “safe,” his spokesman Jason MacDonald said.

The incident came a day after 25-year-old Martin Couture-Rouleau ran over a soldier, killing him before being shot dead by police as he emerged from his wrecked car wielding a knife.

The government branded this a terrorist attack by a suspected militant, amid reports that Couture-Rouleau was a supporter of the so-called Islamic State, a jihadist group operating in Iraq and Syria.

If the driver's alleged jihadist sympathies are confirmed, it would be Canada's first ever militant attack, although authorities have warned they are tracking 90 suspected extremists in the country.

Authorities raised the security threat level from low to medium after the incident, which came as Canadian jets were to join the US-led air armada bombarding militants in Iraq.

“This level means that intelligence has indicated that an individual or group within Canada or abroad has the intent and capability to commit an act of terrorism,” the Public Safety Ministry said.

Outside parliament, police were seen taking cover behind vehicles, as others expanded a cordoned off area to a city block around parliament.