US authorities say a suspected Chinese spy balloon has flown over “a number of sensitive sites” as it remains in the skies over the country’s north.

They include Montana, which is home to fields of underground Minuteman III intercontinental ballistic missile silos, one potential target for Chinese espionage.

Washington has been tracking the balloon since it entered US airspace two days ago, including by observing it with manned military planes.

A senior US defence official told CNN of a “ground stop” at Billings Airport in Montana earlier this week, and discussed the potential “mobilisation of assets, including F-22s (fighter planes).”

“The context for that was, it would put some things on station in the event that a decision was made to bring this down while it was over Montana,” the official said.

“We wanted to make sure we were coordinating with civil authorities to empty out the airspace around that potential area.”

In the end, US defence officials advised President Joe Biden against shooting down the balloon for fear the debris could pose a safety threat.

“We had been looking at whether there was an option yesterday” (Wednesday) bring the aircraft down “over some sparsely populated areas in Montana, but we just couldn’t buy down the risk enough to feel comfortable recommending shooting it down”, one official has told other US media.

It was the “strong recommendation” of senior military leaders, including the Chairman of the Joint Chiefs of Staff General Mark Milley, “not to take kinetic action due to the risk to safety and security of people on the ground from the possible debris field”, CNN reports.

The exact size of the balloon has not been revealed, but it was “large enough to cause damage” if downed. US authorities say the balloon remains in the country, although they have not revealed where.

“Clearly the intent of this balloon is for surveillance,” a senior US defence official said on Thursday (local time).

China and the US – the world’s two largest economies – have experienced tensions of late, clashing over Taiwan and China’s human rights record and its military activity in the South China Sea.

Washington has been tracking the balloon since it entered US airspace two days ago, including by observing it with manned military aircraft.

Officials declined to say how high it was flying but acknowledged it was operating above civilian air traffic and below outer space.

US officials brought up the issue with their Chinese counterparts.

“We have communicated to them the seriousness with which we take this issue,” the official said.

“We have made clear we will do whatever is necessary to protect our people and our homeland.”

Spy balloons have flown over the US several times in recent years, but this balloon appeared to be lingering longer than in previous instances.

“Currently, we assess that this balloon has limited additive value from an intelligence collection perspective but we are taking steps nevertheless to protect against foreign intelligence collection of sensitive information,” the official said.

-with AAP