According to high-ranking defense sources, it is relatively certain that it is a Chinese high-altitude balloon that flew over sensitive locations to gather information.
One of the places over which the balloon was observed is Montana, home to a military base with nuclear weapons, among other things.
The Pentagon has been tracking the balloon for several days since it made its way over the northern United States, according to Defense Force spokesman Patrick Ryder.
"No physical threat"
The U.S. is taking steps to ensure it does not collect sensitive information, Ryder said, adding that the U.S. military decided not to shoot the balloon down.
- Currently it travels at an altitude well above commercial air traffic and does not pose a military or physical threat to people on the ground, says Ryder.
The US has alerted China through several channels and communicated the seriousness of the incident, he adds.
Similar events with balloons have been observed in recent years, according to Ryder.