Moments before the decision, China issued a rare statement of regret and blamed winds for blowing over what it called a civilian airship.

But President Joe Biden's administration was not impressed and, with the rival Republican Party already on the offensive, Blinken postponed his two-day visit that would have started Sunday.

"We have noted the PRC statement of regret but the presence of this balloon in our airspace is a clear violation of our sovereignty as well as international law and it is unacceptable," said a senior US official, referring to the People's Republic of China.

The official said Blinken personally conveyed the decision and would still go ahead with his visit to Beijing once "conditions are right," although no date was set.

The official voiced confidence that the delay would not set back communication between the two powers, a top goal of the trip.

"Candidly speaking in this current environment, I think it would have significantly narrowed the agenda that we would have been able to address," he told reporters.

Blinken would have been the first top US diplomat to visit China since October 2018, signaling a thaw following intense friction under former president Donald Trump.

Last month, Blinken said he would use the trip to help establish "guardrails" to prevent the relationship from escalating into all-out conflict.

Republican lawmakers quickly pounced on the balloon incident, casting Biden -- who has largely preserved and at times expanded Trump's hawkish policies on China -- as weak.

"President Biden should stop coddling and appeasing the Chinese communists. Bring the balloon down now and exploit its tech package, which could be an intelligence bonanza," tweeted Senator Tom Cotton, a prominent hardliner who had urged Blinken to call off his trip.

- China voices regret -

The Pentagon said Thursday it was tracking the balloon which flew far above the western state of Montana and decided for safety reasons not to shoot it down.

After initial hesitation, Beijing admitted ownership of the "airship" and said it veered off course due to the winds.

"The airship is from China. It is a civilian airship used for research, mainly meteorological, purposes," said the statement attributed to a foreign ministry spokesperson.

"The Chinese side regrets the unintended entry of the airship into US airspace due to force majeure," it said, using the legal term for an act outside of human control.

"The Chinese side will continue communicating with the US side and properly handle this unexpected situation."

The senior official who announced Blinken's trip delay said the United States has not changed its assessment that it was a spy balloon.

A US defense official said earlier that Biden had asked for military options but that the Pentagon believed shooting the object down would put people on the ground at risk from debris.

The balloon has "limited additive value from an intelligence collection perspective," the defense official told reporters on condition of anonymity.

The United States is also widely believed to spy on China, although generally with more advanced technology than balloons.

The northwestern United States is home to sensitive airbases and nuclear weapons in underground silos.

Pentagon spokesman Pat Ryder said that, as of Thursday, the balloon was "traveling at an altitude well above commercial air traffic."

"It does not present a military or physical threat to people on the ground," Ryder said in a statement.

- Bracing for worst -

Defense Secretary Lloyd Austin, who was visiting the Philippines, held discussions Wednesday with top Pentagon officials after Biden asked for options.

In the Philippines, Austin agreed to expand the US military presence, weeks after a separate troop deal with another regional ally, Japan.

The US military moves show that the United States is preparing for potential conflict over Taiwan, the self-governing democracy China claims as its own, despite diplomatic efforts.

Biden held a surprisingly cordial meeting in November with Chinese President Xi Jinping on the sidelines of a summit in Bali, where they agreed to send Blinken to Beijing.

A US military officer recently told his forces to be ready for war with China.

"I hope I am wrong. My gut tells me we will fight in 2025," Air Mobility Command chief General Mike Minihan wrote in a memo, saying that US elections in 2024 would also "offer Xi a distracted America."

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