The United States military is tracking a suspected Chinese surveillance balloon over the country and has not shot it down due to the danger of injuring people on the ground.
A senior US defence official, speaking on the condition of anonymity, said that there was a "very high confidence" it is a Chinese high-altitude balloon and it was flying over sensitive sites to collect information.
One of the locations it was spotted was in Montana, which is home to one of the nation's three nuclear missile silo fields at Malmstrom Air Force Base.
Brig Gen Patrick Ryder, Pentagon press secretary, said 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."
He added it is not the first time that similar balloons have been seen in recent years.
Joe Biden was briefed with Defence Secretary Lloyd Austin and Army Gen Mark Milley, chairman of the Joint Chiefs of Staff, advising against taking "kinetic action" because of risks to the safety of people on the ground, said another senior official, speaking anonymously.
At the same time the US has "engaged" Chinese officials and communicated the seriousness of the matter.
It comes as Secretary of State Antony Blinken was due to make his first trip to Beijing this weekend, to try to find some common ground amidst rising tensions.
Although the trip has not been formally announced, both Beijing and Washington have been talking about his imminent arrival.
Fighter jets were readied to shoot the balloon down if the order came from the White House but the Pentagon advised against it as although it was over a sparsely populated area of Montana, its size would create a debris field large enough to put people at risk.
The spy balloon was trying to fly over the Montana missile fields, but the US has assessed that it has "limited" value in terms of providing China intelligence it couldn't already collect by other means, such through spy satellites, said a senior defence official.
The official said the balloon was large enough that despite its high altitude, commercial pilots could see it and all air traffic was halted at Montana's Billings Logan International Airport from 1:30 p.m. to 3:30 p.m. Wednesday.
Montana Gov Greg Gianforte said he was briefed about the situation after the Montana National Guard was notified of an ongoing military operation taking place in Montana airspace.
"From the spy balloon to the Chinese Communist Party spying on Americans through TikTok to CCP-linked companies buying American farmland, I'm deeply troubled by the constant stream of alarming developments for our national security," Gov Gianforte said.
Meanwhile, House Speaker Kevin McCarthy, tweeted, "China's brazen disregard for U.S. sovereignty is a destabilizing action that must be addressed."
Tensions with China are particularly high on numerous issues, ranging from Taiwan and the South China Sea to human rights in China's western Xinjiang region and the clampdown on democracy activists in Hong Kong.
Beijing has also increased preparations for a potential blockade or military action against Taiwan, which has stirred increasing concern among military leaders, diplomats and elected officials in the U.S., Taiwan's key ally.
