He said he took the threat by the Somali-based group al-Shabab seriously.
In a video, the group urged followers to carry out attacks on shopping centres in the US, Canada and the UK.
Al-Shabab was responsible for the 2013 attack on Westgate shopping mall in Nairobi that killed 67 people.
Mr Johnson told CNN that the threat was part of "a new phase" of terrorism in which attacks would increasingly come from "independent actors in their homelands".
"Anytime a terrorist organisation calls for an attack on a specific place, we've got to take that seriously," he said.
In the video, a man with a British-sounding accent and full face covering calls on supporters of al-Shabab to attack "American or Jewish-owned" Western shopping centres. (+ BBC video) » | Sunday, February 22, 2015
