A video purported to be by Somalia’s al-Qaeda-linked rebel group al-Shabab has urged Muslims to attack shopping malls in the US, Canada, Britain and other Western countries.

The threat came in the final minutes of a more than hour-long video in which the extremists also warned Kenya of more attacks like the September 2013 assault on the Westgate Mall in Nairobi in which 67 people were killed.

The video included footage from major news organisations showing the assault on the mall and said it was in reprisal for alleged abuses by Kenyan troops against Muslims in Somalia.

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The masked narrator concluded by calling on Muslims to attack shopping malls, specifically naming the Mall of America in Minnesota, as well as the West Edmonton Mall in Canada and the Westfield mall in Stratford, England.

The authenticity of the video could not be immediately verified.

Speaking on morning talk shows in the US, Department of Homeland Security Secretary Jeh Johnson called the video “the new phase” of the global terrorist threat and warned the public to be vigilant.

“These groups are relying more and more on independent actors to become inspired, drawn to the cause and they’ll attack on their own,” Johnson said, speaking on CNN’s State of the Union.

“I am very concerned about serious potential threats of independent actors here in the United States. We’ve seen this now in Europe, we’ve seen this in Canada.”

In the slickly produced video posted online, the narrator, his face wrapped in a black-and-white kaffiyeh-type scarf and wearing a camouflage jacket, spoke with a British accent and appeared to be of Somali origin.

“What if such an attack were to occur in the Mall of America in Minnesota? Or the West Edmonton Mall in Canada? Or in London’s Oxford Street?” the man said, then called for Britain’s Westfield mall to be targeted.

While al-Shabab has carried out attacks in neighbouring Kenya, Uganda and Djibouti, which all have troops fighting the extremists as part of the multi-national African Union force, the al-Qaeda affiliate has never operated outside East Africa and the Horn of Africa.

In Kenya, the government dismissed the al-Shabab video.

“They’re using propaganda to legitimise what cannot be legitimised,” Interior Ministry spokesman Mwenda Njoka said.

“When you lead a group to go and attack a shopping mall and kill innocent shoppers that cannot be legitimised, those were not soldiers.”