Shoppers heading for the Mall of America in Minnesota would see added security and should be "particularly careful," Homeland Security Secretary Jeh Johnson said Sunday after a video calling for attacks on major shopping centers surfaced this weekend.

"Americans should still feel that they are free to associate, they are free to go to public gatherings...I would say that if anyone is planning to go to the Mall of America today, they've got to be particularly careful," Mr. Johnson said on CNN Sunday morning.

The hourlong video, calling for attacks on malls in the U.S., Canada and London, purportedly came from al-Shabaab, the Somali-based terrorist organization that had claimed responsibility for the 2013 attack on the Westgate Shopping Mall in Nairobi, Kenya, that left 67 people dead.

"There will be enhanced security there that will be apparent," Mr. Johnson said. "But public vigilance, public awareness and public caution in situations like this is particularly important, and it's the environment we're in, frankly."

He also said he was "very concerned about the serious potential threat of independent actors here in the U.S. Any time a terrorist organization calls for an attack on a specific place, we've got to take that seriously."

Mr. Johnson, who made the rounds of the Sunday morning news shows, focused on the congressional impasse over funding the Department of Homeland Security. He said the GOP-led Congress would jeopardize national security if it withholds funding to block President Barack Obama's executive action shielding millions of illegal immigrants from deportation. The department's funding expires at midnight Friday.

A joint statement issued by the police department of Bloomington, Minn., the Federal Bureau of Investigation, the Minnesota Department of Public Safety and others said that "there is no credible threat associated with Mall of America," and that law-enforcement officials along with the Mall of America would "evaluate this and other information as it becomes available."

British police said they were aware of al-Shabaab's video appeal for sympathizers to lay armed siege to London's biggest shopping centers, including the twin Westfield malls that lie to the east and west of the city.

"Counterterrorist officers are assessing the content of the footage," said a Metropolitan Police spokeswoman. She declined to comment on what, if any, additional security measure had been taken in response to the threat.

The U.K. government said on Sunday that it doesn't comment on specific threats or intelligence matters. It added that the U.K.'s assessment of the threat of a terror attack--which is set by a body independent of government--is already at "severe", the second-highest level on its five-point scale.

"That means that a terrorist attack is [seen as] "highly likely" and the public should remain vigilant, but there is no intelligence to suggest one is imminent," the U.K.'s Home Office said in a statement.

The terror threat level was raised in August in response to the rise of Islamist militancy in Syria and Iraq. The move triggers certain heightened security measures such as a more visible armed police presence is some areas.

The British government has said it takes seriously the threat from online extremist propaganda and has been working with the Internet industry to remove such material. Since December 2013, 56,000 pieces of extremist material have been taken down from the Internet, it said.

Westfield Corp., the Australian firm that owns the two malls, wasn't immediately reachable for comment.

Mall of America said in a statement that it was aware of a video, which included a mention and images of the mall, located in Bloomington, Minn., and is taking extra security precautions.

"We will continue to monitor events with the help of federal, state and local law enforcement agencies," the statement said.

One sales woman at the Mall of America said traffic was "soft" like a normal Sunday, but added that it was -8 degrees outside, which could keep even some Minnesotans at home.

In targeting a U.S. shopping mall, terrorists hope to hurt consumer spending, the growth engine of the U.S. economy. "No question, [the threat] is a disruption to economic activity," said Diane Swonk, chief economist at Mesirow Financial.

But she said the impact on consumer spending may be less than anticipated. "Malls were already losing out to online shopping. You can still shop. You just do it from the comfort of your home," she said.

In addition, Ms. Swonk said "any disruption will be transitory. People revert back to normal patterns over time." She pointed out that happened after the Sept. 11, 2001, terrorist attacks.

The National Retail Federation, a trade group for retailers, offered reassurances to shoppers.

"In light of the security alert for U.S. malls, retailers are moving quickly to implement their top level emergency plans. That includes alerting personnel and shoppers to maintain a heightened level of vigilance at all times," said Senior Vice President Bill Thorne.

He added that shoppers "should rest assured that retailers have been and remain prepared for any emergency that may arise."

Write to Louise Radnofsky at louise.radnofsky@wsj.com and Kathleen Madigan at kathleen.madigan@wsj.com

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