U.S. President Barack Obama voiced American sympathy and offered help as U.S. officials warned of the grave risks of attacks by Islamic jihadis and their sympathizers.
Hours after gunfire erupted on Parliament Hill and a Canadian soldier was slain at the National War Memorial, Mr. Obama called Prime Minister Stephen Harper to express "the American people's solidarity with Canada," the White House announced.
Later on Wednesday, in the Oval Office, Mr. Obama said "obviously the situation there is tragic," referring to the attack in Ottawa. But the President was careful not to call the shootings an act of terrorism or say whether the attack in Canada was part of a larger campaign.
"We don't yet have all the information about what motivated the shooting," Mr. Obama said during an availability originally arranged to discuss the Ebola strategy. "We don't yet have all the information about whether this was part of a broader network or plan, or whether this was an individual or series of individuals who decided to take these actions. But it emphasizes the degree to which we have to remain vigilant when it comes to dealing with these kinds of acts of senseless violence or terrorism."
The U.S. Federal Bureau of Investigation issued a notice to its field officers and said it was ready "to assist our Canadian partners."
"Although there is no specific reporting indicating a threat to the United States," the FBI urged "vigilance in light of recent calls for attacks against government personnel by terrorist groups and like-minded individuals."
In Washington, fears of counterattacks by Islamic jihadis have increased in recent weeks following Mr. Obama's decision to launch a U.S.-led bombing campaign against militants who have seized a vast swath of Iraq and Syria.
Jihadi sympathizers and so-called foreign fighters – Westerners who have joined the Islamic State and may return to their homelands and strike – have been the focus of U.S. concerns.
"The United States and our allies have been very mindful of the risks posed by foreign fighters, individuals who have Western passports and have travelled to the region to take up arms alongside ISIL," said White House spokesman Josh Earnest, referring to the Islamic State by its former name, the Islamic State of Iraq and the Levant.