WASHINGTON — The nation’s top homeland security official urged shoppers on Sunday to be “particularly careful” in visiting the Mall of America, a sprawling shopping center outside Minneapolis, after it was targeted for a terrorist attack in a video posted by a radical Islamist group.

“If anyone is planning to go to the Mall of America today, they have got to be particularly careful,” Jeh Johnson, the secretary of Homeland Security, said on CNN.

“There will be enhanced security there,” 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.”

Mall of America officials said they had tightened security after the Shabab, an extremist group in Somalia that is linked to Al Qaeda, posted a 75-minute video on Saturday. The Shabab claimed responsibility for the 2013 attack on the Westgate shopping mall in Nairobi, Kenya, in which more than 60 people were killed.