World leaders offered their condolences and support to Canada following Wednesday's shootings in Ottawa, as the story captured international attention.

South of Canada's border, American networks went into live coverage mode and White House reporters turned their attention to the story, asking officials what effect the events in Canada are having on the United States.

U.S. President Barack Obama was briefed on the shootings by his top homeland security adviser Wednesday morning and spoke by phone with Prime Minister Stephen Harper a short time later. 

Obama told reporters Wednesday afternoon that he offered his condolences on behalf of Americans to the family of the soldier killed and to all Canadians.

"Obviously the situation there is tragic," Obama said, referring to the hit-and-run incident that killed another soldier, Warrant Officer Patrice Vincent, on Monday and to the shooting today.

'We are going to do everything we can to make sure we are standing side-by-side with Canada during this difficult time,    ' - U.S. President Barack Obama

Obama said that while there are unanswered questions about the motive and who carried out the attack, "it emphasizes the degree to which we have to remain vigilant when it comes to dealing with these kinds of senseless acts of violence, or terrorism."

The U.S. president said he pledged in his call with Harper to make sure that national security teams in both countries are working closely together, saying Canada is not only an ally of the U.S. but a friend and neighbour.

"It's very important, I think, for us to recognize that when it comes to dealing with terrorist activity that Canada and the United States have to be entirely in sync. We have in the past, I am confident we will continue to do so in the future."

Obama said Harper appreciated the expressions of concern from Americans, and the president went on to recall his own visit to Parliament Hill and said he was reminded of how warmly he was received there and "how wonderful the people there were."

"So obviously we are all shaken by it, but we are going to do everything we can to make sure we are standing side-by-side with Canada during this difficult time," said Obama.

Harper-Obama

Prime Minister Stephen Harper speaks by phone with Obama about the shootings in Ottawa. (Carl Vallee/Prime Minister's Office)

White House press secretary Josh Earnest said American officials were in touch with their Canadian counterparts and had offered assistance, but he couldn't specify whether the FBI and CIA specifically had made calls. The FBI, however, said the following on Twitter: "We stand ready to assist our partners as they deal with the ongoing situation in their capital."

"Canada is one of the closest friends and allies of the United States and from issues ranging from the strength of our NATO alliance to the Ebola response to dealing with ISIL, there is a strong partnership and friendship and alliance between the United States and Canada," said Earnest. "The United States strongly values that relationship and that relationship makes the citizens of this country safer."

Security heightened at Arlington memorial

Earnest said he was not aware of the threat level being raised in the United States as a result of what happened in Ottawa or any added security measures at the White House or on Capitol Hill.

CNN reported that security was increased at Arlington National Cemetery, not far from downtown Washington, because of the events in Ottawa. The cemetery, where rows upon rows of white headstones mark the graves of deceased military members, features a tomb of the unknown soldier that is also guarded by members of the military, much like the National War Memorial in Ottawa.

CNN reported that officials were not disclosing what specific measures were being implemented to boost security.

Earnest was asked repeatedly about any possible connection with Canada's participation in the U.S.-led international coalition to fight ISIS in Iraq and Syria but he did not want to speculate.

"The circumstances around today's tragic events in Canada are still unknown," he said, but noted that Canada and the United States and other allies have been working together to try and prevent their citizens from going abroad to join ISIS, or ISIL as the group is also known. Canada has been "robustly engaged" in that effort, he said.

"We certainly are appreciative of the commitment that Canada has made to this broader international effort alongside some 60 other countries," he said about the campaign to defeat ISIS.

Republican John Boehner, Speaker of the House of Representatives, issued a statement late Wednesday night saying the House of Representatives offers its full support to the people of Canada and its government, after the deaths of two soldiers.

Boehner said Harper, and his counterpart, House of Commons Speaker Andrew Scheer, "will continue to have friends and allies in the U.S. Congress who are prepared to stand shoulder to shoulder with them against our common enemies and who are prepared to move forward in our aims to improve the quality of life and economic opportunities for both our countries."

"The world is growing more complex and dangerous by the day, but we will not be intimidated by those who seek violence, repression, and fear," Boehner went on to say. "Our common cause and fight — dignity and freedom for all — will always persevere, even during the darkest of times."

Britain's Cameron 'appalled'

U.S. Secretary of State John Kerry issued a statement saying he spoke with his counterpart, Foreign Affairs Minister John Baird Wednesday evening, expressed his condolences and committed full support to Canada in response to the "heinous and evil attacks."

"The United States has faced this kind of violence first-hand on our own soil, and we grieve with Canada, seared by the memory of our own painful experiences," Kerry's statement said. Kerry offered praise for first responders who quickly acted to protect MPs and the public.

"We have great confidence in Canadian law enforcement and security forces and stand ready to assist in any way. We will continue to work closely with our Canadian colleagues to ensure the safety of all our people, and together to counter violent extremism in North America and elsewhere around the world," said Kerry.

The U.S. Embassy in Ottawa was put on lockdown earlier in the day, according to a State Department spokeswoman.

In a similar security increase, Australian Prime Minister Tony Abbott said that his country's federal police have stepped-up patrols around parliament and at the Canadian High Commission in Canberra, the capital. 

United Nations Secretary General Ban Ki-moon also said his thoughts were with Canadians.

"The secretary general is aware of the situation unfolding in Ottawa, including in the Canadian Parliament, during which a soldier has reportedly been shot. He hopes the situation will be brought quickly under control by Canadian law enforcement authorities. His thoughts are with the people and government of Canada at this difficult time," a statement provided to CBC News read.

British Prime Minister David Cameron was another world leader who offered his support to Canada. "I'm appalled by today's attack in Ottawa," he wrote on Twitter, adding that he is offering his full support to Harper "and the Canadian people as they deal with this incident."

British Foreign Minister Philip Hammond said on Twitter he was "horrified" to learn of the shootings. 

"Our thoughts are with you," he wrote in response to a tweet from his counterpart, Canada's Foreign Affairs Minister John Baird.

The Prime Minister's Office said on Twitter that Harper also spoke by phone with Israeli Prime Minister Benjamin Netanyahu about the shootings.

Gen. Chuck Jacoby, commander of NORAD, issued a statement that offered condolences and talked about the bond between Canada and the United States. 

"My heart goes out to our Canadian comrades on this tragic day following the shootings in Ottawa, as well as the hit-and-run attack in Quebec earlier this week, both of which targeted Canadian Forces personnel," it said. "We offer our sincerest condolences to the people of Canada.  We share their grief, but also their determination not to be intimidated by these cowardly acts." 

The Chinese embassy condemned the attacks in a statement and sent condolences to the family of the soldier killed. New Zealand Prime Minister John Key wrote on Twitter that his country stands in solidarity with Canada.