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 defence 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 nations 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.

He said similar balloon activity has been seen in the past several years. He added that the U.S. took steps to ensure it did not collect sensitive information.

The defense official said the U.S. has engaged Chinese officials through multiple channels and communicated the seriousness of the matter.

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

The U.S. is expanding its military presence in Asia, in a string of moves aimed at countering Beijing and reassuring Indo-Pacific allies that America will stand with them against threats from China and North Korea.

The senior defence official said the U.S. did get fighter jets, including F-22s, ready to shoot down the balloon if ordered to by the White House. The Pentagon ultimately recommended against it, noting that even as the balloon was over a sparsely populated area of Montana, its size would create a debris field large enough that it could have put people at risk.

The official would not specify the size of the balloon, but said it was large enough that despite its high altitude, commercial pilots could see it.

The official said what concerned them about this launch was the altitude the balloon was flying at and the length of time it lingered over a location, without providing specifics.