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.

Brig. Gen. Patrick Ryder, Pentagon press secretary, provided a brief statement on the issue, saying the government continues to track the balloon. He said it is “currently traveling at an altitude well above commercial air traffic and does not present a military or physical threat to people on the ground.”