“Clearly the intent of this balloon is for surveillance,” a senior US defence official told reporters on Thursday.

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.

Senior US military leaders considered shooting the balloon down over Montana but eventually recommended against it because of the potential safety risk from debris.

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.

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

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.

-AAP