It's a campaign that threatens more and more countries, he said.
"And frankly, this terrorist caliphate in our judgment represents an increasing long-term threat to the security of our own country. It is that serious," Harper said Wednesday during a brief stop in British Columbia on his annual trip to the North.
"We have been in ongoing contact with our principal allies, and we will be announcing some additional steps that we will be able to take with them in the days ahead."
Harper made the comments during what was to be a photo op at a business in Chilliwack, B.C. He did not elaborate on the measures during a campaign-style speech to supporters in Langley on Wednesday night.
Islamic State militants beheaded Foley and posted a grisly video online entitled "Message to America."
Foley was killed in retaliation for air strikes ordered by Obama against the Islamic State militants in Iraq. The group has undertaken a brutal campaign in both Iraq and neighbouring Syria.
In Washington, American President Barack Obama denounced the group earlier Wednesday as a "cancer" threatening the entire region, affirming that U.S. will not scale back its military presence despite the threat from IS to kill another American hostage.
Foley, 40, went missing in northern Syria in November 2012 while freelancing for Agence France-Presse and the Boston-based media company GlobalPost. He had not been heard from since.
The video of his murder was posted online on Tuesday and drew international condemnation.
Germany announced it would supply the Kurds in Iraq with weapons to fight the insurgents. Italy's defence minister said the country hopes to contribute machine-guns, ammunition and anti-tank rockets.
French Foreign Minister Laurent Fabius called the Islamic State a "caliphate of barbarism."
British authorities are investigating to determine whether the man who killed Foley was a British citizen.
The journalist's mother, Diane Foley, said her son was courageous to the end and called his death "just evil."
Since Aug. 8, there have been nearly 90 U.S. airstrikes in Iraq on targets of the advancing Islamic State, including security checkpoints, vehicles and weapons caches.
- With files from the Associated Press