Separately, Canada’s defence ministry said a “high-altitude surveillance balloon” was detected and that it was monitoring a “potential second incident”, without giving further details, adding that it was in frequent contact with the United States.

The Canadian statement did not reference China.

American officials have declined to say why the US believed the balloon belonged to China, saying only that the US had high confidence in the assessment. The US has engaged Chinese officials through multiple channels and communicated the seriousness of the matter.

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Chinese foreign ministry spokesperson Mao Ning said Beijing was “verifying” the situation.

“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,” she told a regular daily briefing in Beijing on Friday.

Spy balloons have flown over the United States several times in recent years, but this balloon appeared to be lingering longer than in previous instances, one of the US officials said.

China has often complained about US surveillance by ships and spy planes near its own territory, leading to occasional confrontation over the years.

“Clearly, the intent of this balloon is for surveillance, and the current flight path does carry it over a number of sensitive sites,” said a US official, who spoke on condition of anonymity.

But the Pentagon did not believe it constitutes a particularly dangerous intelligence threat.

“We assess that this balloon has limited additive value from an intelligence collection perspective,” the official said.

US officials became aware of the balloon earlier in the week. Reuters reported that the balloon had been tracked near the Aleutian Islands and Canada before entering the United States.

Unlike satellites, which require space launchers that cost hundreds of millions of dollars, balloons can be launched cheaply. But the technology doesn’t offer any intelligence-gathering capability beyond what China’s low-orbit satellites already provide.

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Balloons are not directly steered, but can be roughly guided to a target area by changing altitudes to catch different wind currents, according to a 2005 study for the Air Force’s Airpower Research Institute.

Retired US Colonel Steve Ganyard told ABC News that it was possible the balloon drifted, rather than being nefariously deployed. He said intentionally deploying a spy balloon over the US would be highly provocative, with little value.

Pentagon spokesman Pat Ryder said the balloon was travelling 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.

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,” Rubio said on Twitter.

Republican House Speaker Kevin McCarthy said he would request a “Gang of Eight” briefing, referring to a classified national security briefing for congressional leaders and Republican and Democratic leaders of the intelligence committees.

The balloon’s presence comes amid slowly simmering tensions between the United States and China on numerous issues, ranging from Taiwan and the South China Sea to human rights in China’s Western Xinjiang region and the clampdown on democracy activists in Hong Kong.

On Thursday, CIA Director William Burns called China the “biggest geopolitical challenge” currently facing the United States.

Agence France-Presse, Reuters, Bloomberg, Associated Press