The Pentagon said on Thursday that it was tracking a Chinese spy balloon flying high over the United States that appeared to be carrying out a surveillance of highly sensitive nuclear weapons sites, according to US media reports.

Fighter jets were mobilised but military leaders advised President Joe Biden against shooting the balloon out of the sky for fear debris could pose a safety threat to people on the ground.

“The United States government has detected and is tracking a high-altitude surveillance balloon that is over the continental United States right now,” a Reuters report cited Pentagon spokesperson Brigadier General Patrick Ryder as saying.

An official said the balloon flew over the US northwest where there are sensitive airbases and strategic missiles in silos underground, according to an AFP report.

“Clearly, the intent of this balloon is for surveillance, and the current flight path does carry it over a number of sensitive sites,” he said.

It was flying at a high enough altitude not to threaten commercial aviation, the official added.

But the Pentagon did not believe it constitutes a particularly dangerous intelligence threat.

“We assess that this balloon has limited additive value from an intelligence collection perspective,” the official said.

The United States took “custody” of the balloon when it entered U.S. airspace and had observed it with piloted U.S. military aircraft, one the official told reporters on condition of anonymity.

The incident comes just days ahead of U.S. Secretary of State Antony Blinken’s planned trip to Beijing.