A video released by the Somalian Al-Shabaab militant group has named London's Oxford Street and two of its major shopping malls as specific targets for terrorist attacks.

Released on Saturday, the video features a masked man speaking with an English-sounding accent. He urges followers to attack Oxford Street and the Westfield shopping centres in Stratford and Shepherd's Bush, as well as several US and Canadian shopping malls, even giving the GPS co-ordinates.

"We call upon our Muslim brothers, particularly those in the West, to answer the call of Allah and target disbelievers wherever they are," the militant says in the 76-minute clip.

"If just a handful of mujahedeen fighters can bring Kenya to a complete standstill for nearly a week, imagine what a dedicated mujahedeen in the West could do to American or Jewish owned shopping centres across the world?" he added, referring to the September 2013 attack on the Westgate shopping mall in Nairobi, which resulted in the deaths of at least 67 people. Al-Shabaab, which is linked to al-Qaeda, claimed responsibility for that attack.

"What if such an attack on the Mall of America in Minnesota or the West Edmonton mall in Canada or in London's Oxford Street, or any of the hundred or so of the Jewish owned Westfield shopping centres? Take the Westfield shopping centre in Stratford or White City for example, what would be the implications of such an attack, one can only imagine.

"All it takes is a man with firm determination, of which our Muslim ummah [community] has plenty of."

The chief of homeland security in the US, Jeh Johnson, confirmed that authorities were taking the threats seriously, and urged citizens to be "particularly careful".

"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," he told CNN.

"We're beyond the phase now where these groups would send foreign operatives into countries after being trained someplace."

A spokesperson for the London Metropolitan Police said the terrorism command was assessing the content of the video.