US Homeland Security Secretary Jeh Johnson has urged people to be vigilant following a terror threat to Western shopping centres, including one of America's largest malls.
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.
But Mr Johnson added that he was not advising people not to go to the malls named by the militants.
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.
He specifically mentions Minnesota's Mall of America - the second-largest US shopping centre - and Canada's West Edmonton Mall, as well as London's Oxford Street and the UK capital's two Westfield shopping centres.
Co-ordinates for the various targets were listed on the screen as they were described.
The BBC's Naomi Grimley in Washington says it is possible that the video is part of a rivalry between al-Shabab, which is linked to al-Qaeda, and Islamic State, which has dominated media coverage recently.
UK police said they were aware of the video and assessing the contents.
Both Mall of America and West Edmonton Mall have issued statements saying they were implementing additional security measures.
Minnesota is home to a large Somali population and a Minnesota man was indicted last week on charges of conspiring to support Islamic State.
Police and security services in Canada, France and Denmark have been on high alert recently following attacks by so-called "home-grown" terrorists inspired by groups such as al-Shabab and Islamic State.