The US Homeland Security Department has said that it is not aware of any specific plot against US shopping malls, backing away from comments by the department's chief that he takes seriously a threat by Somali-based armed group al-Shabab against shopping sites in Western countries.

Some US and Canadian officials earlier on Sunday had cast doubt on the credibility of the threat made in a video attributed to al-Shabab, which appeared to call for attacks on Western shopping areas, specifically mentioning Mall of America in Minnesota, the West Edmonton Mall in Canada, London's Oxford Street and sites in Paris.

But Homeland Security Secretary Jeh Johnson said that "anytime a terrorist organisation calls for an attack on a specific place, we've got to take that seriously", and told shoppers to be careful without providing any more detail.

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

But hours later, Homeland Security Department spokeswoman Marsha Catron said the department and the FBI had shared information about the video with local law enforcement and "private sector partners."

"As a general matter, however, we are not aware of any specific, credible plot against the Mall of America or any other domestic commercial shopping centre," she said in a statement.

One US intelligence official meanwhile said that security officials indeed are worried about the risk of an attack on US soil by an individual working alone, but al-Shabab as a group has not appeared to gain much traction with most Somalis in Western countries, including in Minneapolis where there is a sizable population of Somali origin.

"In balance, I don't think this video adds much on top of the ubiquitous 'lone offender' threat," Reuters news agency quoted the official, who spoke on condition of anonymity, as saying.

Staff Sergeant Brent Meyer of Canada's Royal Canadian Mounted Police said "there is no evidence at this time of any specific or imminent threat to Canadians". In Britain, a spokesman said London police were aware of the video and were assessing it.

Mall of America and West Edmonton Mall issued statements saying they were implementing extra security measures anyway.

Source: Reuters