A huge, high-altitude Chinese balloon sailed across the U.S. on Friday, drawing severe Pentagon accusations of spying on sensitive military sites despite China's firm denials. Secretary of State Antony Blinken abruptly canceled a high-stakes Beijing trip aimed at easing U..S.-China tensions.

Aside from the government response, fuzzy videos dotted social media as people with binoculars and telephoto lenses tried to find the “spy balloon” in the sky as it headed southeastward over Kansas and Missouri at 60,000 feet (18,300 meters).

It was spotted earlier over Montana, which is home to one of America’s three nuclear missile silo fields at Malmstrom Air Force Base, Defense officials said.

The U.S. actually had been tracking the balloon since at least Tuesday, when President Joe Biden was first briefed, White House press secretary Karine Jean-Pierre told reporters. According to three U.S. officials, Biden was initially inclined to order the surveillance balloon to be blown out of the sky, and a senior defense official said the U.S. had prepared fighter jets, including F-22s, to shoot it down if ordered.

The officials, who spoke on the condition of anonymity to discuss internal deliberations, said Defense Secretary Lloyd Austin and Gen. Mark Milley, chairman of the Joint Chiefs of Staff, strongly advised Biden against shooting down the balloon, warning that its size — as big as three school buses — and considerable weight could create a debris field large enough to endanger Americans on the ground. The Pentagon also assessed that after unspecified U.S. measures, the possibility of the balloon uncovering important information was not great.

It was not the first time Chinese surveillance balloons have been tracked over U.S. territory, including at least once during former President Donald Trump’s administration, officials said.

Blinken's trip cancellation came despite China’s claim that the balloon was merely a weather research “airship” that had blown off course. The Pentagon rejected that out of hand — as well as China's contention that the balloon was not being used for surveillance and had only limited navigational ability.

Blinken, who had been due to depart Washington for Beijing late Friday, said he had told senior Chinese diplomat Wang Yi in a phone call that sending the balloon over the U.S. was “an irresponsible act and that (China's) decision to take this action on the eve of my visit is detrimental to the substantive discussions that we were prepared to have.”

After passing the sensitive military sites in Montana, the balloon was moving southeastward over the heartland of the central United States during the day and was expected to remain in U.S. airspace for several days, officials said.

The development marked a new blow to already strained U.S.-Chinese relations that have been in a downward spiral for years over numerous issues. Still, U.S. officials maintained that diplomatic channels remain open and Blinken said he remained willing to travel to China “when conditions allow.”

“We continue to believe that having open lines of communication is important," he said.

Biden declined to comment on the matter when questioned at an economic event. Two likely 2024 reelection challengers, Trump, and Nikki Haley, the former South Carolina governor and U.N. ambassador, said the U.S. should immediately shoot down the balloon.

Several Republican congressmen said the same, and a number blasted the administration for “allowing” the balloon intrusion.

“The idea that Communist China has a spy balloon headed towards Whiteman Air Force Base in Missouri right now — the home of the Stealth Bomber — is absolutely unbelievable,” said Sen. Eric Schmitt, R-Mo. “No American should accept this. I don’t.”

Jean-Pierre did not shed light on why the administration waited until Thursday to make its concerns public.

Brig. Gen. Pat Ryder, Pentagon press secretary, refused to say on Friday whether there was any new consideration of shooting the balloon down. He said it currently was posing no threat.

Ryder said it was maneuverable, not just at the mercy of the wind, and had changed course.

Still, weather experts said China’s claim that the balloon had gone off course was not unfeasible. China’s account of wind patterns known as the Westerlies carrying a balloon to the western United States was “absolutely possible - not possible, likely,” said Dan Jaffe, a professor of atmospheric chemistry at the University of Washington.

As for Blinken's trip, Jean-Pierre said a diplomatic visit to China was not appropriate at such a time. She said that "the presence of this balloon in our airspace ... is a clear violation of our sovereignty as well as international law and it is unacceptable this occurred.”

A State Department official said Blinken and Deputy Secretary of State Wendy Sherman had both protested to the top official at the Chinese Embassy on Wednesday, a day before the Pentagon announced the discovery of the balloon.

Blinken’s long-anticipated meetings with senior Chinese officials had been seen in both countries as a possible way to find some areas of common ground at a time of major disagreements over Taiwan, human rights, China’s claims in the South China Sea, North Korea, Russia’s war in Ukraine, trade policy and climate change.

Although the trip, which was agreed to in November by Biden and Chinese President Xi Jinping at a summit in Indonesia, had not been formally announced, officials in both Beijing and Washington had spoken in recent days about Blinken’s imminent arrival for meetings on Sunday and Monday.

China, which angrily denounces surveillance attempts by the U.S. and others over areas it considers to be its territory and once forced down an American spy plane and held its crew captive on Hainan Island, was relatively conciliatory in its response to the U.S. complaints.

In a statement that approached an apology, the Chinese foreign ministry said the balloon was a civilian airship used mainly for meteorological research. It said said the airship had limited “self-steering” capabilities and had “deviated far from its planned course” because of winds.

“The Chinese side regrets the unintended entry of the airship into U.S. airspace due to force majeure,” the statement said, citing a legal term used to refer to events beyond one’s control.