US Secretary of State Antony Blinken has told a top Chinese official that it was "irresponsible" of Beijing to send a surveillance balloon over US soil as he explained why he postponed a visit to China.

In a phone call with Wang Yi, Mr Blinken noted China's "statement of regret but conveyed that this is an irresponsible act and a clear violation of US sovereignty and international law that undermined the purpose of the trip," a State Department statement said.

Mr Blinken's visit, which had been expected to start today, was postponed after the balloon was tracked flying across the United States.

White House spokesperson Karine Jean-Pierre said President Joe Biden was briefed on the balloon flight on Tuesday and there was an administration "consensus that it was not appropriate to travel to the People's Republic of China at this time."

China earlier expressed regret that what it called a "civilian" airship had strayed into US territory after being blown off course.

Pentagon spokesperson Air Force Brigadier General Patrick Ryder told reporters yesterday that the government was tracking a high-altitude surveillance balloon over the continental United States.

He said it was "traveling at an altitude well above commercial air traffic and does not present a military or physical threat to people on the ground".

US military leaders considered shooting down the balloon over Montana on Wednesday but eventually President Joe Biden decided against it because of the safety risk from debris, US officials said.

Republican Senator Tom Cotton had called for Mr Blinken to cancel his China trip, while Republican former president Donald Trump, a declared presidential candidate for 2024, posted "SHOOT DOWN THE BALLOON!" on his Truth Social media platform.

In a statement today, China's foreign ministry said the balloon was for civilian meteorological and other scientific purposes and that it regretted that the airship had strayed into US airspace.

It said it would continue to communicate with the United States to "properly handle" the unexpected situation.

A Chinese government spokesperson said earlier that "China has no intention of violating the land territory and airspace of any sovereign country".

The US said it had raised the matter with their Chinese counterparts through diplomatic channels.

"We have communicated to them the seriousness with which we take this issue," a US official said.

Chinese President Xi Jinping meeting US President Joe Biden last year

The postponement of Mr Blinken's trip, which was agreed to in November by President Biden and Chinese President Xi Jinping, will be a blow to those on both sides who saw it as an overdue opportunity to stabilise an increasingly fractious relationship. The last visit by a US secretary of state was in 2017.

China is keen for a stable US relationship so it can focus on its economy, battered by the now-abandoned zero-Covid policy and neglected by foreign investors alarmed by what they see as a return of state intervention in the market.

In recent months, President Xi has met world leaders, seeking to re-establish ties and settle disagreements.

Relations between China and the US have soured in recent years, particularly following then-US House Speaker Nancy Pelosi's visit to Taiwan in August, which prompted Chinese military drills near the self-ruled island.

Since then, Washington and Beijing have sought to communicate more frequently and prevent ties from worsening.

One US official said the balloon was assessed to have "limited additive value from an intelligence collection perspective".

The United States took "custody" of the balloon when it entered US airspace and had observed it with piloted military aircraft, one US official said.

One American official said the flight path would carry the balloon over a number of sensitive sites, but did not give details. Malmstrom Air Force Base in Montana is home to 150 intercontinental ballistic missile silos.

The news initially broke yesterday as CIA Director William Burns was speaking at an event at Washington's Georgetown University, where he called China the "biggest geopolitical challenge" facing the United States.

US Senator Marco Rubio, the top Republican on the Senate intelligence committee, said the spy balloon was alarming but not surprising.

"The level of espionage aimed at our country by Beijing has grown dramatically more intense & brazen over the last 5 years," Mr Rubio said on Twitter.

Billings-Logan International Airport in Montana issued a ground stop as the military mobilised assets, including F-22 fighter jets, in case Mr Biden ordered that the balloon be shot down.

Defence expert John Parachini estimated the size of the balloon was equivalent to three bus lengths.

Such balloons typically operate at 24,000m-37,000m, well above where commercial air traffic flies.

The highest-performing fighter aircraft typically do not operate above 19,810m although spy planes such as the U-2 have a service ceiling of 24,850m or more.

Craig Singleton, a China expert with the Foundation for Defense of Democracies, said that such balloons had been widely used by the US and Soviet Union during the Cold War and are a low-cost intelligence gathering method.