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.
Donald Trump says that when he takes power next month he will direct the US Justice Department to “vigorously pursue” the death penalty.
The US president-elect, 78, said he would do so to protect Americans from what he called “violent rapists, murderers and monsters”.
Mr Trump was responding to President Joe Biden’s decision to commute the sentences of almost all federal inmates on death row – whom Mr Trump called “37 of the worst killers in our country”.
“When you hear the acts of each, you won’t believe that he did this. Makes no sense,” Mr Trump posted on his social media platform Truth Social.
“Relatives and friends are further devastated. They can’t believe this is happening!”
He continued: “As soon as I am inaugurated, I will direct the Justice Department to vigorously pursue the death penalty to protect American families and children from violent rapists, murderers, and monsters.
“We will be a Nation of Law and Order again!”
President Biden, 82, announced on Monday that he would reduce the sentences of 37 of the 40 federal death row prisoners to life in prison without the possibility of parole, saying he was “guided by my conscience and my experience as a public defender”.
The three others the president did not spare are Robert Bowers, who fatally shot 11 people at a Pittsburgh synagogue in 2018; Dylann Roof, who gunned down nine black churchgoers in Charleston, South Carolina, in 2015; and Dzhokhar Tsarnaev, who carried out a 2013 bombing at the Boston Marathon that killed three people and injured almost 300 others.
‘I condemn these murderers’
Despite sparing the lives of 37, Mr Biden added: “Make no mistake: I condemn these murderers, grieve for the victims of their despicable acts, and ache for all the families who have suffered unimaginable and irreparable loss.”
During Mr Trump’s first term in office between 2017 and 2021, the US Justice Department put 13 federal inmates to death.
He has since said he would like to expand capital punishment to include child rapists, migrants who kill US citizens and law enforcement officers, and those convicted of drug and human trafficking.
Mr Biden, who ran for president opposing the death penalty, put federal executions on hold when he took office in January 2021.
His latest decisions come after a coalition of criminal justice advocacy groups, former prosecutors and business leaders wrote letters to the White House asking for Mr Biden to commute the sentences ahead of Mr Trump’s inauguration on 20 January.
Pope Francis also appealed to Mr Biden, who is Catholic, to reduce the sentences to imprisonment.
Unlike executive orders, clemency decisions cannot be reversed by a president’s successor, although the death penalty can be sought more aggressively in future cases.
Denmark has announced plans to boost its defence spending for Greenland with a “stronger presence in the Arctic” – a few hours after Donald Trump repeated his call for the US to buy the vast island.
Danish defence minister Troels Lund Poulsen said the package would amount to a “double-digit billion amount” in krone, or at least $1.5bn (£1.2bn).
He told the Jyllands-Posten newspaper the money would be used to buy two inspection ships, two long-range drones and two sled dog teams as well as more personnel for Denmark’s Arctic Command in the capital Nuuk.
Denmark will also upgrade the Kangerlussuaq Airport so that it can handle F-35 fighter jets.
Greenland, which sits between the Atlantic and Arctic oceans, is 80% covered by an ice sheet and is home to a large US military base.
The world’s biggest island, whose capital is closer to New York than the Danish capital Copenhagen, has mineral, oil and natural gas wealth.
But development has been slow, leaving its economy reliant on fishing and annual subsidies from Denmark.
“For many years, we have not invested sufficiently in the Arctic, now we are planning a stronger presence,” Mr Poulsen said.
He called the timing of the announcement an “irony of fate”, coming just hours after Mr Trump’s latest comments on purchasing the territory.
With the Pituffik air base, Greenland is strategically important for the US military and its ballistic missile early-warning system.
Greenland defiant
The president-elect sparked anger on the territory when he wrote that American ownership and control of the island was an “absolute necessity” for “purposes of national security and freedom throughout the world”.
Its prime minister Mute Egede hit back, saying: “Greenland is ours. We are not for sale and will never be for sale. We must not lose our long struggle for freedom.”
And Danish defence minister Mr Poulsen said: “My response to Trump is the same as the prime minister’s. Greenland does not want to exchange the Commonwealth for other relations. But that is up to Greenland itself.”
Mr Trump also proposed buying Greenland during his first term in office – an idea the Danish Prime Minister Mette Frederiksen called “absurd”.
Greenland has been part of Denmark for more than 600 years and gained autonomy from the country in 1979.
Under Greenland’s self-government act, enacted by Denmark and Greenland in 2009, Greenlanders are recognised as a people or nation entitled to the right of self-determination, with the option of independence.
On Monday, in an announcement naming Ken Howery as his ambassador to Denmark, Mr Trump wrote: “For purposes of national security and freedom throughout the World, the United States of America feels that the ownership and control of Greenland is an absolute necessity.”
He has also threatened to take back control of the Panama Canal, accusing Panama of charging excessive rates to use the waterway, which allows ships to cross between the Pacific and Atlantic oceans.
American Airlines was forced to ground all flights in the US on Christmas Eve due to an unspecified technical issue.
The airline did not immediately say why it was stopping all flights, but social media was quickly abuzz with travellers worrying about getting to their loved ones for the holiday.
A groundstop notice was lifted not long after it was issued, but the possibility of disruption remains with so many flights needing to make up time.
Earlier on Tuesday, the airline said on social media: “An estimated timeframe has not been provided, but they’re trying to fix it in the shortest possible time.”
The Federal Aviation Agency said American Airlines was reporting “a technical issue and has requested a nationwide ground stop”.
In an update on Tuesday afternoon it said: “American Airlines reported a technical issue this morning and requested a nationwide ground stop. The ground stop has now been lifted.”
Passengers on social media reported having their flights stuck on the runway at various airports and being sent back to the gate.
American Airlines operates thousands of flights per day to more than 350 destinations in more than 60 countries.