The head of US Northern Command, Admiral William Gortney, ordered the heightened alert to make sure troops have a "greater awareness and vigilance out in the field" for potential security risks, a defense official told AFP.
"We want to make sure they're taking extra care guarding installations and our people," the official said.
The move raised the official alert status one notch higher on a five-point scale, from the fourth level "Alpha" to the third level "Bravo," which the military describes as an "increased and predictable threat of terrorism."
The move came after two men attempted to storm an exhibition on Sunday showing caricatures of the Prophet Mohammed in a Dallas suburb. The two heavily-armed gunmen were shot dead by a local police officer before they could approach the building where the event was being held.
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