Intelligence officials tell NBC News that Canadian authorities have heard would-be terrorists discussing potential ISIS-inspired “knife and gun” attacks against U.S. and Canadian targets inside Canada.

While intelligence officials say some of the discussions raise the possibility of attacks on U.S. citizens and other U.S. and Western targets in Canada, they have not picked up talk of anyone crossing the border into the U.S. to stage attacks. They also stressed that the possible attacks were at the "aspirational" stage.

Intelligence officials say that Canadian authorities are monitoring "hundreds of people" in Canada who have either gone to Syria to link up with ISIS and other Islamist groups and returned, or who have tried and failed to make the trip. The unknowns, those inspired by ISIS but as yet on no radar, add to the worry, helping to make this what U.S. and Canadian officials have said is the most dangerous time since 9/11 for a domestic terror attack.

Both U.S. and Canadian officials fear the beheading of an innocent person in a public place, or the slashing of citizens on a crowded street until police arrive to shoot and “martyr” the terrorists.

In a statement, the press secretary for Canada’s Minister of Public Safety told NBC News, "While I cannot comment on operational matters of national security, I can say that security agencies are constantly evaluating the terrorist threat and taking action to protect Canadians."

Canadian officials are weighing increased security around public buildings in coming days, government officials there say. The increased concern is prompted in part by Tuesday's vote in the Canadian parliament to join the coalition of nations carrying out airstrikes against ISIS targets in Iraq.

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"If left unchecked this terrorist organization will grow and grow quickly. They have already voiced their local and international terrorist intentions and identified Canada as a potential target," said Prime Minster Stephen Harper.

Intelligence analysts in Canada have also been studying the latest ISIS propaganda video, which includes several minutes of an English-speaking man, wearing a mask, addressing the camera.

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The FBI on Tuesday posted an excerpt of the video on its own website, www.fbi.gov, and asked for public help in identifying the man. His accent indicates he could be from the US or Canada, the FBI said.

Canadian intelligence officials have also analyzed the video. They say the man's accent suggests he could be the son of immigrants and that English could be his second language. Voice recognition software, they say, failed to come up with a match to any known terrorism suspects.

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They also say that while the man's face is mostly covered by his mask, enough is visible that a close relative would be able to recognize him.

Both U.S. and Canadian officials have received dozens of tips about the man 's identity. None have panned out so far.