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The US is tracking a suspected Chinese high-altitude surveillance balloon over the continental United States, defense officials said on Thursday, a discovery that risks adding further strain to tense US-China relations.

Pentagon spokesman Brig. Gen. Patrick Ryder said the US government has been tracking the balloon for several days as it made its way over the northern United States, adding it was “traveling at an altitude well above commercial air traffic and does not present a military or physical threat to people on the ground.”

Speaking on background, a senior US defense official said senior military officials had advised President Joe Biden not to shoot it down due to fear the debris could pose a safety threat to people on the ground.

“We are confident that this high-altitude surveillance balloon belongs to the [People’s Republic of China],” the senior defense official said. “Instances of this activity have been observed over the past several years, including prior to this administration.”

While the balloon’s current flight path carries it over “a number of sensitive sites,” the official said it does not present a significant intelligence gathering risk. The balloon is assessed to have “limited additive value” from an intelligence collection perspective, the official added.

The US, the official said, is “taking steps nevertheless to protect against foreign intelligence collection of sensitive information. We are also tracking what abilities it could have in gaining insights, and continue to monitor the balloon as it was over the continental United States.”

The US believes Chinese spy satellites in low Earth orbit are capable of offering similar or better intelligence, limiting the value of whatever Beijing can glean from the high-altitude balloon, which is the size of three buses, according to another defense official.

“It does not create significant value added over and above what the PRC is likely able to collect through things like satellites in low Earth orbit,” the senior defense official said.

The US government has engaged with the Chinese government both through the Chinese embassy in Washington and the US diplomatic mission in China, according to the official.

US national security officials have constantly warned about Chinese espionage efforts and the balloon’s presence in the US comes at a sensitive moment with Secretary of State Antony Blinken expected to travel to Beijing in the coming days, a significant trip meant to follow up on President Joe Biden’s meeting with Chinese President Xi Jinping last year.

Biden has declared China “America’s most consequential geopolitical challenge” and competition between the two global superpowers is intense. Tensions have flared in recent years over the self-governing island of Taiwan, China’s human rights record and its military activities in the South China Sea, among a host of other issues.

The president has been briefed on the balloon’s movements and requested military options on how to deal with it, according to a senior administration official.

Biden took Milley’s advice not to order the balloon shot down and the official stressed that it does not pose a military threat emphasizing that the administration acted “immediately” to protect against the collection of sensitive information.

The senior defense official mentioned reports from Wednesday about a “ground stop” at Billings Airport in Montana, and the “mobilization of assets, including F-22s.”

“The context for that was, it would put some things on station in the event that a decision was made to bring this down while it was over Montana,” the official said. “So we wanted to make sure we were coordinating with civil authorities to empty out the airspace around that potential area.”

However, it was ultimately the “strong recommendation” of senior military leaders, including the Chairman of the Joint Chiefs of Staff Gen. Mark Milley, not to shoot it down due to the risk to safety of people on the ground.

“Why not shoot it down? We have to do the risk-reward here,” the official said. “So the first question is, does it pose a threat, a physical kinetic threat, to individuals in the United States in the US homeland? Our assessment is it does not. Does it pose a threat to civilian aviation? Our assessment is it does not. Does it pose a significantly enhanced threat on the intelligence side? Our best assessment right now is that it does not. So given that profile, we assess the risk of downing it, even if the probability is low in a sparsely populated area of the debris falling and hurting someone or damaging property, that it wasn’t worth it.”

Montana is home to fields of underground Minuteman III intercontinental ballistic missile silos, one potential target for Chinese espionage.

The senior defense official said on Thursday that if the risk level changes, the US “will have options to deal with this balloon.”

We have communicated to [Chinese officials] the seriousness with which we take this issue. … But we have made clear we will do whatever is necessary to protect our people and our homeland.”

This story is been updated with additional developments.