The US has been monitoring a suspected Chinese surveillance balloon that has been spotted in the skies over the northern US this week.
Pentagon officials said in a briefing they are "confident that this high-altitude surveillance balloon" belongs to China.
US military chiefs have for now decided against blowing the alleged spy craft out of the sky, citing safety concerns.
US President Joe Biden has been briefed on the situation, said the Pentagon.
The defence official speaking on condition of anonymity said the US government did prepare fighter jets, including F-22s, in case the White House ordered officials to shoot down the balloon.
The object was spotted on Wednesday over Montana, the sparsely populated western US state that is home to a nuclear missile base.
Pentagon officials said that US military leaders, including Defence Secretary Lloyd Austin and General Mark Milley, chairman of the US Joint Chiefs of Staff, met on Wednesday to assess the threat. Mr Austin was travelling in the Philippines at the time.
The military commanders advised against taking "kinetic action" against the balloon because of the danger of debris falling to the ground.
The senior defence official said there was no "significantly enhanced threat" of US intelligence being compromised because American officials "know exactly where this balloon is and exactly where it's passing over".
He added that there was also no threat to civilian aviation as the balloon was "significantly" above the altitude used by commercial airlines.
The senior defence official said the US had raised the matter with Chinese officials in their embassy in Washington DC and in Beijing.
"We have communicated to them the seriousness with which we take this issue," the official said.
The Chinese embassy in Washington DC did not immediately respond to a request for comment.
The object flew over Alaska's Aleutian Islands and through Canada before appearing over the city of Billings, Montana, on Wednesday, officials say.
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During Thursday's briefing at the Pentagon, officials declined to disclose the aircraft's current location. They also refused to provide more details of the object, including its size.
"There have been reports of pilots seeing this thing even though it's pretty high up in the sky," the unnamed defence official said.
"So you know, it's, it's sizable."
The official added that similar balloons have appeared in the skies over the US in the past few years.
But the current balloon "is appearing to hang out for a longer period of time" and has been "more persistent" than previous spy crafts.
It is unclear why it was seen in that particular region of the US. Billings is around 250 miles (400km) southwest of the Malmstrom Air Force Base, which is one of three US air bases that house the nuclear-capable Minuteman III intercontinental ballistic missile.
Pentagon officials refused to confirm if the base was a surveillance target, but agreed "the current flight path does carry it over a number of sensitive sites".
The alleged spy craft confounded social media users in Montana on Wednesday.
People posted photos of a pale round object in the sky that floated separately from the moon. Others reported seeing US military planes in the area, apparently monitoring the object.
It also led to a two-hour ground stop at Billings Logan International Airport after aviation officials made the decision to close 50 sq miles of air space.
It comes ahead of US Secretary of State Antony Blinken's visit to China.
The top US diplomat will be in Beijing next week to hold talks on a wide range of issues, including security, Taiwan and Covid-19.
He will meet Chinese President Xi Jinping, the Financial Times newspaper reported on Thursday.
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