The U.S. homeland security chief said on Sunday he takes seriously a threat made by Somali-based Islamist militants against shopping malls, including the Mall of America in Minnesota, and urged people going there to be careful.

Homeland Security Secretary Jeh Johnson was reacting to a video released by al Shabaab appearing to call for attacks on Western shopping areas, specifically mentioning Mall of America, the West Edmonton Mall in Canada and London's Oxford Street.

"This latest statement from al Shabaab reflects the new phase we've evolved to in the global terrorist threat, in that you have groups such as al Shabaab and ISIL publicly calling for independent actors in their homelands to carry out attacks," Johnson told the CNN program "State of the Union," using an acronym for the militant group Islamic State.