Bejing says it is working to verify reports of a Chinese spy balloon flying over the United States and warns against "hyping up the issue".

"I want to emphasize that until the facts are clear, making speculations and hyping up the issue are not conducive to solving the problem properly," says Chinese foreign ministry spokeswoman Mao Ning.

The discovery of the aircraft comes just days before a scheduled visit to China by US Secretary of State Antony Blinken.