A suspected Chinese spy balloon has been flying over the United States for a couple of days, US officials said yesterday, in what would be a brazen act just days ahead of a planned trip to Beijing by US Secretary of State Antony Blinken.

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, advice Biden accepted, US officials said.

The United States took "custody" of the balloon when it entered US airspace and observed it with piloted US military aircraft, one of the officials told reporters on condition of anonymity.

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

Blinken is expected to travel to China next week for a visit agreed to in November by Biden and Chinese President Xi Jinping. It was not clear how the discovery of the spy balloon might affect those plans.

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

US 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," a US official said.

US 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.

His fellow Republican, Senator Tom Cotton, called for Blinken to cancel his trip.

Republican House Speaker Kevin McCarthy said he would request a "Gang of Eight" briefing, referring to a classified national security briefing for congressional leaders and Republican and Democratic leaders of the intelligence committees.

The Biden administration briefed Gang of Eight staff earlier yesterday and has offered additional briefings, a US official said.

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.

Relations between China and the United States have soured in recent years, particularly following then-US House Speaker Nancy Pelosi's visit to Taiwan in August, which prompted dramatic Chinese military drills near the self-ruled island.

Since then, Washington and Beijing have sought to communicate more frequently and prevent ties from worsening.

Potential safety risk

US 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 mobilised 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.

A separate US official said the spy balloon had been tracked near the Aleutian Islands and Canada before entering the United States.

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

Limited intelligence value?

Such balloons typically operate at 24,000-37,000m, well above where commercial air traffic flies. The highest-performing fighter aircraft typically do not operate above 65,000 feet, although spy planes such as the U-2 have a service ceiling of 80,000 feet or more.

Craig Singleton, a China expert of the Foundation for Defense of Democracies, said such balloons had been widely used by the United States and Soviet Union during the Cold War and are a low-cost intelligence gathering method.

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, an 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.

Singapore-based security analyst Alexander Neill said while the balloon was likely to provide a fresh irritant to China-US ties, it was probably of limited intelligence value compared to other elements China’s modernising military has at its disposal.

"China has its own constellation of spy and military satellites that are far more important and effective in terms of watching the US, so I think it is a fair assumption that the intelligence gain is not huge,” said Neill, who is an adjunct fellow at Hawaii’s Pacific Forum think-tank.

- Reuters