It flew over the Aleutian Islands, off the coast of Alaska, and through Canadabefore entering the US.
The balloon is still in US airspace but officials declined to say where it is now.
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They acknowledged it was operating above civilian air traffic and below “outer space”, but declined to say how high it was flying.
Military and defence leaders have considered shooting the balloon out of the sky but decided against it due to the safety risk from falling debris.
US defence secretary Lloyd Austin convened a meeting of senior military and defence leaders to review the threat profile of the balloon and possible responses, which were presented to US President Joe Biden on Wednesday.
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2:17
US tracks suspected Chinese balloon
The US has engaged Chinese officials “with urgency” and communicated the seriousness of the situation.
Pentagon press secretary Brigadier General Patrick Ryder said: “The United States government has detected and is tracking a high-altitude surveillance balloon that is over the continental United States right now.
“The US government to include NORAD (North American Aerospace Defence Command), continues to track and monitor it closely.
“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.
“Instances of this kind of balloon activity have been observed previously over the past several years.
“Once the balloon was detected, the US government acted immediately to protect against the collection of sensitive information.”
China and the US have experienced tensions of late, clashing over Taiwan and China’s human rights record and its military activity in the South China Sea.
US Secretary of State Antony Blinken is expected to travel to China in the coming days.
It is not clear if this will affect his travel plans, which the State Department has not formally announced.
Spy balloon threatens efforts to ease US-China relations
Distrust between the Chinese and the Americans is as high as it’s been for decades.
An incident like this would serve to feed that distrust no matter when it happened, but coming, as it has, just days before Secretary of State Antony Blinken’s highly significant visit to Beijing could seriously undermine tentative efforts being made on both sides to try to halt any further deterioration in relations.
Mr Blinken is expected to land in Beijing on Sunday and had planned to meet his opposite number Qin Gang as well as Wang Yi, China’s highest ranking diplomat.
A huge amount of painstaking diplomatic effort will have gone into making such a visit possible – the fact it was happening at all is a progress of sorts.
In recent days there has even been suggestions Mr Blinken might meet with President Xi Jinping himself.
If so, he would be the first US secretary of state granted this level of access in five years and it would be a major sign both sides are serious about attempting to smooth over their deeply damaged relations.
The Chinese leader and US President Joe Biden both recognised when they met at the G20 summit late last year that they need to do more to ensure that their distrust and competition does not descend into conflict and confrontation.
This visit was a clear part of that effort. But mutual recognition that spiralling tensions aren’t a good thing is not the same thing as the active rebuilding of trust.
This incident will likely be seen by the Americans as flying in the face of both. And there is, perhaps, an awareness here in Beijing of just how much jeopardy this incident poses to those fledgling efforts.
Indeed, at a regular news conference in Beijing on Friday, there was a clear desire on the Chinese part to contain speculation.
Foreign ministry spokesperson Mao Ning said China was “verifying” the situation and added: “I would like to emphasise that until the facts are clarified, speculation and hype will not be helpful to the proper resolution of the issue.”
Given the low ebb of current relations between the two, Mr Blinken’s visit was not expected to deliver any breakthroughs. It was being framed more as a chance for both sides to restate their positions and red lines and keep the channels of dialogue open.
It will likely never be known if this spy balloon was purposefully scheduled ahead of the visit or if it’s just unfortunate timing, but if it forces Mr Blinken to cancel, the ramifications for the longer term project of containing deteriorating relations could be very serious indeed.