The U.S. is tracking a suspected Chinese surveillance balloon that has been spotted over U.S. airspace for a couple days, but the Pentagon decided not to shoot it down due to risks of harm for people on the ground, officials said Thursday.

A senior defense official told Pentagon reporters that the U.S. has “very high confidence” it is a Chinese high-altitude balloon and it was flying over sensitive sites to collect information. One of the places the balloon was spotted was Montana, which is home to one of the nation’s three nuclear missile silo fields at Malmstrom Air Force Base.

The official spoke on condition of anonymity to discuss sensitive information.

The Pentagon announcement comes days before Secretary of State Antony Blinken is expected to travel to China. It’s not clear if this will affect his travel plans, which the State Department has not formally announced.