The Pentagon said Thursday it was tracking a Chinese spy balloon flying high over the northwest United States, reviving tensions between the two countries just days ahead of a rare visit to Beijing by top US diplomat Antony Blinken.

"Verification is under way" over the reports, foreign ministry spokeswoman Mao Ning told a regular briefing, adding that "until the facts are clear, making conjectures and hyping up the issue will not help to properly resolve it".

"China is a responsible country and always abides strictly by international law. We have no intention of violating the territory or airspace of any sovereign country," she said.

"(We) hope that both sides will handle (the situation) with mutual calm and prudence," she added.

A senior US defence official told reporters Thursday that shooting the balloon down had been considered, but that it had been decided doing so would endanger too many people on the ground.

"Clearly, the intent of this balloon is for surveillance," added the official, who spoke on condition of anonymity.

The official said that the balloon had flown over areas that contained sensitive airbases and nuclear missiles in underground silos, but that the Pentagon did not believe it constituted a particularly dangerous intelligence threat.

The discovery comes just days before an expected visit to China by US Secretary of State Blinken, with managing heightened tensions between the two powers at the top of the agenda.

At the briefing, Mao said she had no information to share on Blinken's visit.