An alleged spy balloon spotted over the US is a Chinese "civilian airship" which had deviated from its planned route, China says.

US defence officials said they believed the balloon, seen above sensitive areas in recent days, was in fact a "high-altitude surveillance" device.

But in a statement, China's foreign ministry said it was used for "mainly meteorological" purposes.

China "regrets the unintended entry" of the balloon into US airspace, it added.

The object flew over Alaska's Aleutian Islands and through Canada before appearing over the city of Billings in Montana on Wednesday, US officials said.

Montana is home to some of the US's nuclear missile silos.

The US decided not to shoot down the balloon because of the danger posed by falling debris, and the limited use of any intelligence the device could gather, the US defence official said.

However, the government prepared fighter jets in case the object had to be shot down.

The Chinese statement said the balloon had been blown off-course by unexpected winds.

"Affected by the Westerlies and with limited self-steering capability, the airship deviated far from its planned course.

"The Chinese side regrets the unintended entry of the airship into US airspace due to force majeure."

The statement referred to the incident as an "unexpected situation" and said Beijing would continue to communicate with the US side.

The balloon has been reported in US media as being about the size of three buses.

During Thursday's briefing at the Pentagon, US defence officials declined to disclose the aircraft's current location and did not give information on where it was launched from.

They added that the balloon was "appearing to hang out for a longer period of time" than others tracked by the US over the past several years.

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