WASHINGTON -- The chaos in Canada's capital attracted the attention of people around the world Wednesday, including U.S. President Barack Obama who received a briefing on the developments from the Oval Office.

The U.S. government increased security in different locations, including at its embassy in Ottawa and reportedly also at its own tomb of the unknown soldier at Arlington National Cemetery near downtown Washington.

Obama spoke by phone with Prime Minister Stephen Harper hours after a masked gunman shot an honour guard at the National War Memorial.

A spokesman for the U.S. president began the White House daily briefing with a word of condolence, as news of the incident spread around the world.

"Before I go to your questions, there is some sad news out of Canada today I just wanted to talk about," said Obama's press secretary Josh Earnest.

"The thoughts and prayers of everybody here at the White House go out to the families of those who were affected by today's shooting in Canada, as well as to the family of the soldier who was killed earlier this week."

He said officials throughout the U.S. government had been in close touch with their counterparts in Canada -- which he described as a valued friend and partner.

British Prime Minister David Cameron took to Twitter to express his indignation at the events unfolding in Canada: "I'm appalled by today's attack in Ottawa. I offer my full support to (at)pmharper and the Canadian people as they deal with this incident."

Meanwhile, the Canada-U.S. North American Aerospace Defence Command said it had taken steps so that it could respond should there be any incidents involving aviation.

Down the street from the White House, on Pennsylvania Avenue, the Canadian embassy denied a U.S. media report that it had been placed on lockdown. A spokeswoman for the embassy said that, following news of the shooting, staff there simply locked the embassy's front door.

The shooting quickly became a top news story in numerous countries.

All the main American news networks offered live coverage throughout the day. It was also the lead item on the best-known newspapers in a number of countries including the U.S., the UK, Mexico, France and Italy.

The top headline on France's Le Monde newspaper website was, "Ottawa Boucle" (Ottawa On Lockdown). Italy's La Repubblica website ran numerous headlines underneath one that declared, "Canada Sotto Attacco" (Canada Under Attack).

Even in Turkey -- which borders an active war zone where an international coalition is fighting Islamist rebels -- the Canadian events became a top news item.

A number of Canadian politicians who witnessed the scenes on Parliament Hill appeared on American news shows to describe what they'd seen. The NDP's Charlie Angus described hearing the gunshots from inside the Opposition members' chamber during the weekly caucus meeting.

Angus told Fox News that Canadians weren't going to be intimidated by this type of act.

"We're not gonna let punks, or crazies, or terrorists, take us down. Not gonna happen," the NDP MP told the cable-news outlet.

"We don't give in to it."

One U.S. law-enforcement analyst said this incident will prompt the same kind of scrutiny that occurred when a knife-wielding man recently jumped the fence to enter the White House.

That incident prompted a series of follow-up stories about other security lapses, which led to the resignation of the head of the Secret Service.

A recurring theme of some of the American news coverage was that question of security: How did a gunman manage to walk into Canada's Parliament?