A suspected Chinese spy balloon, said to be the size of three buses, was spotted over the United States' airspace, the Pentagon has said, a development that is likely to further strain the already tense bilateral ties ahead of Secretary of State Antony Blinken's key visit to Beijing.

Pentagon spokesman Brig. Gen. Patrick Ryder said the US government has been tracking the balloon for several days as it made its way over the northern United States and flew over sensitive installations.

He told reporters on Thursday that the balloon was "travelling at an altitude well above commercial air traffic and does not present a military or physical threat to people on the ground."

"NORAD (North American Aerospace Defense Command) continues to track and monitor it closely," he said, adding the balloon was spotted over Montana on Thursday and is "said to be the size of three buses".

Montana, a sparsely populated state, is home to one of only three nuclear missile silo fields in the US, at Malmstrom Air Force Base.

"Once the balloon was detected, the US government acted immediately to protect against the collection of sensitive information," Ryder said.

Reacting to the Pentagon's report, a Chinese Foreign Ministry spokesperson confirmed that the balloon was a Chinese "civilian airship" which had deviated from its planned route.

"The airship is from China. It is a civilian airship used for research, mainly meteorological, purposes," the spokesperson said in a statement posted on the Chinese foreign ministry's website.

"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 said.

Earlier, Chinese foreign ministry spokesperson Mao Ning told reporters in Beijing on Friday that China was verifying the Pentagon's report on the balloon.

She urged calm "until the facts are clear."

"China is a responsible country and always abides strictly by international law. We have no intention of violating the territory or airspace of any sovereign country," she said.

In Washington, Pentagon spokesman Ryder said the balloon is travelling at an altitude well above commercial air traffic and does not present a military or physical threat to people on the ground.

US President Joe Biden was briefed about the incident, a senior defence official said, adding the Pentagon is exploring various options.

Chairman of the Joint Chiefs of Staff Gen Mark Milley and US Northern Command Gen Glen VanHerck have recommended not to "take kinetic action" because of the potential risk to the safety and security of people on the ground from the possible debris field, officials said.

"Currently, we assess that this balloon has limited additive value from an intelligence collection perspective. But we are taking steps nevertheless to protect against foreign intelligence collection of sensitive information," said a defence official.

The US government has engaged with the Chinese government both through the Chinese embassy in Washington and the US diplomatic mission in China.

US national security officials have constantly warned about growing Chinese espionage efforts and the balloon’s presence in the US comes at a sensitive moment with Secretary of State Blinken expected to travel to Beijing next week, a significant trip meant to follow up on President Joe Biden’s meeting with Chinese President Xi Jinping in November last year in Bali, Indonesia.

Biden has declared China "America's most consequential geopolitical challenge" and competition between the two major powers are growing.

Bilateral tensions have flared over the self-governing island of Taiwan, China’s human rights record and its military activities in the South China Sea, among a host of other issues.

Meanwhile, Canada said on Thursday that it is also tracking the Chinese balloon's movements and working with its American partners.

Balloons are one of the oldest forms of surveillance technology. Compared to other air types of surveillance devices, unmanned balloons can be operated cheaply.

The US believes Chinese spy satellites in low Earth orbit are capable of offering similar or better intelligence, limiting the value of whatever Beijing can glean from the high-altitude balloon, CNN quoted a defence official as saying.