US security chief Jeh Johnson's 'new phase of terror' warning

The head of US homeland security has warned Americans to be vigilant after a terror threat to major shopping centres.

Jeh Johnson said he took the threat by the Somalia-based, al Qaeda-affiliated 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.

Mr Johnson told ABC that the threat was part of "a new phase" of terrorism.