An unnamed government official told NBC News' Pete Williams that the Department of Homeland Security based the warning on "very new" intelligence, though it wasn't aware of any "specific threat."
"It's real. It's real and we got very good information," an anonymous government source told CNN Wednesday. "It's based on a credible source. We're taking it seriously so are other countries taking it very seriously."
"While we are not aware of a specific threat to the homeland at this time, this routine communication is an important part of our commitment to making sure we meet that priority," a senior U.S. official told ABC News. "As always, our security apparatus includes a number of measures, both seen and unseen, and DHS will continue to adjust security measures to fit an ever evolving threat environment."
Homeland Security said in a statement that "regularly shares information with domestic and international partners, including those associated with international events such as the Sochi Olympics."
As security concerns mounted, Russian officials banned liquids of any size on Sochi-bound flights last month.
The Winter Olympics start Friday in Sochi.
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