The FBI has been staging mock attacks at American shopping malls to test its readiness after a siege rocked an upscale complex in Nairobi, Kenya, two years ago.

The simulations were conducted during off hours, and the bureau worked with shopping centers to improve security, an unnamed official said, CNN reports.

Al-Shabaab militants killed more than 60 people when they attacked an the Nairobi mall in September 2013. The Somalia-based jihadi terrorist group called for similar attacks on shopping centers in the United States, the United Kingdom and Canada in a video released on Saturday.

Last year, the FBI worked with a number of malls across the United States to increase preparedness. The fake attacks it staged at the shopping facilities were designed test the readiness of SWAT teams and related units.

Saturday's threat, meanwhile, will not trigger a highly visible increase in security measures, Jesse Tron, spokesman for the International Council of Shopping Centers, told USA Today.

"There won't be any mad dash or scramble to improve security because security is constantly evolving and improving," Tron said. "Security officials are not reactionary because they have been doing this all along."

Still, Minnesota's Mall of America, which was mentioned as a potential target in the al-Shabaab video, noted it had "implemented extra security precautions."

And visitors at the facility in a Twin Cities suburb noted some changes but would not allow the measures to change their regular shopping habits, NBC News said.

"There's additional security, so I'm just not too worried about (the threat)," Mall of America patron Andrew Leizens said.

Shopper Dave Thibault, meanwhile, underlined his defiance.

"I refuse to let those people change the way I live my life", he said in reference to the militants.

Jim Fernandez heads the Shopping Center Security Terrorism Awareness Training Program at Louisiana State University's Stephenson National Center for Security Research and Training, and he said the key to ensuring customer safety lies in cooperation across various agencies.

"It's not just mall security staff, it's local PD, the FBI -- they are all working on these things every day," Fernandez said.