The Pentagon building is seen in Arlington, Virginia, U.S.
The Pentagon building is seen in Arlington, Virginia, U.S. October 9, 2020. Reuters

A suspected Chinese spy balloon has been flying over the United States for a couple of days, and senior U.S. officials have advised President Joe Biden against shooting it down for fear the debris could pose a safety threat.

The incident recalls the lengths to which Beijing and Washington have been willing to go to spy on each other amid rising tensions between the superpowers.

"The United States government has detected and is tracking a high-altitude surveillance balloon that is over the continental United States right now," Pentagon spokesperson Brigadier General Patrick Ryder told reporters.

"The balloon is currently traveling at an altitude well above commercial air traffic and does not present a military or physical threat to people on the ground."

U.S. Senator Marco Rubio, the top Republican on the Senate intelligence committee, said the spy balloon was alarming but not surprising.

"The level of espionage aimed at our country by Beijing has grown dramatically more intense & brazen over the last 5 years," Rubio said on Twitter.

The news broke as CIA Director William Burns was speaking at an event at Washington's Georgetown University, at which he called China the "biggest geopolitical challenge" currently facing the United States.

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

U.S. Secretary of State Antony Blinken is due to visit China in the coming days.

A senior U.S. defense official, briefing reporters on condition of anonymity, said the United States has had "custody" of the balloon since it entered U.S. airspace a couple of days ago and has observed it with piloted U.S. military aircraft.

U.S. Defense Secretary Lloyd Austin, while traveling to the Philippines, convened a meeting of senior Pentagon officials on Wednesday to discuss the balloon incident.

POTENTIAL SAFETY RISK

U.S. military leaders considered shooting down the balloon over Montana on Wednesday but eventually advised Biden against it because of the safety risk from debris, the official told reporters.

The Billings, Montana airport issued a ground stop as the military mobilized assets including F-22 fighter jets in case Biden ordered the balloon be shot down.

"We wanted to make sure we were coordinating with civil authorities to empty out the airspace around that potential area," the official said.

"But even with those protective measures taken, it was the judgment of our military commanders that we didn't drive the risk down low enough. So we didn't take the shot."

The official said the current flight path would carry the balloon over a number of sensitive sites, but did not give details. Malmstrom Air Force Base in Montana is home to 150 intercontinental ballistic missile silos.

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

U.S. officials brought up the issue with their Chinese counterparts through diplomatic channels in Beijing and in Washington.

"We have communicated to them the seriousness with which we take this issue," the official added.

The Chinese foreign ministry did not immediately respond to a Reuters request for comment.

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

"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.