U.S. Homeland Security Secretary Jeh Johnson says homegrown terrorism by previously unknown individuals is the threat that worries him the most. 

Johnson, in a speech to a business audience in Ottawa today, pointed to last year's Boston Marathon bombings as an example of terrorist threats that are difficult to predict.

Johnson also said cybersecurity is one of his top priorities and a common concern when he talks to his foreign and state counterparts.

"There needs to be an increasing awareness, not so much among the most sophisticated businesses ... the financial services industry, for example, but those downstream in the supply chain who are less sophisticated about cyber 'hygiene,'" Johnson said in a moderated question and answer session after his speech. He also took questions from the audience and reporters.

He added there is "legal uncertainty" around information sharing that needs to be resolved and said he was optimistic U.S. legislators would do so.

Jeh Johson, U.S. Homeland Security Secretary

There is an increasing awareness among corporations of the need for cybersecurity, U.S. Secretary of Homeland Security Jeh Johnson told the Canadian American Business Council in Ottawa Tuesday. Homegrown terror attacks like the Boston bombings remain his greatest concern he said. (Sean Kilpatrick/Canadian Press)

"This is for us, and I believe for both our countries, a national imperative. We face cyber attacks from a range of actors on a daily, hourly basis. It is not just a cyber security threat, it's an ongoing series of attacks," he said.

In his midday remarks to the Canadian American Business Council, he also spoke about measures by the U.S. government to improve the flow of good across the border while maintaining security.

Johnson arrived in Ottawa on Monday for a two-day visit that has included bilateral talks on counter-terrorism with several ministers of Stephen Harper's cabinet.

The meetings come as both Canada and the U.S. face intense pressure to clamp down on the recruitment, fundraising and organizational activities of ISIS in North America.

Officially, Johnson's office billed the trip as an opportunity to "discuss engagement through the Beyond the Border initiative."

That joint effort, intended to make legitimate trade and travel between Canada and the U.S. easier while still managing security threats, has experienced delays and complications.

Aside from Tuesday's speech, Johnson was not expected to make any formal announcements or other public statements while in Ottawa.