Washington DC - Secretary of State Antony Blinken has postponed his trip to China as Washington insisted a Chinese surveillance balloon was spying on the US while Beijing said it was a drifting aircraft.

The visit is to take place at another time as soon as circumstances permit, a senior State Department official said on Friday.

China claimed the aircraft was a civilian airship that was hard to steer and not for espionage purposes, but the Pentagon insisted the Chinese aircraft flying over Montana is a spy balloon.

"We know it's a surveillance balloon," US Defense Department spokesperson Pat Ryder said, without providing further details.

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He made it clear that the Pentagon is standing by its assessment of the incident.

He also said the balloon was highly maneuverable, after Beijing cited difficulties in steering it amid Westerly winds.

The aircraft is expected to continue flying over US territory for several days, he said. The Department of Defense said it considered shooting it down, but decided against it, due to the danger of falling debris.

Several Republican lawmakers are criticizing the Biden administration for not shooting it down.

As tensions over the issue rose, the United Nations warned against further antagonism between the two nations.

"We would not want to see such a situation where tensions will be increasing between Washington and Beijing," UN spokesperson Stephane Dujarric said.

Given the global leadership position of both countries, it was up to them to do everything in their power to resolve the situation, he said.

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He added that the United Nations does not have any of its own information about the incident.

Originally, Blinken was scheduled to leave on Friday, but the delay came after the balloon was spotted by the US military over Montana.

After some delay, the Chinese government conceded the airship came from China.

"It is a civilian airship used for research, mainly meteorological, purposes. Affected by the Westerlies and with limited self-steering capability, the airship deviated far from its planned course," a Foreign Ministry spokesperson said in Beijing.

"The Chinese side regrets the unintended entry of the airship into US airspace due to force majeure. The Chinese side will continue communicating with the US side and properly handle this unexpected situation caused by force majeure," the spokesperson said, using a legal term for uncontrollable events.

Washington said it had noted the Chinese statement of regret, "but the presence of this balloon in our airspace is a clear violation of our sovereignty as well as international law."

The State Department official called it "unacceptable that this has occurred. After consultations with our interagency partners, as well as with Congress, we have concluded that the conditions are not right at this moment for Secretary Blinken to travel to China."

Provisionally, F-22 fighter jets were also put on standby, according to the report, and air traffic was temporarily suspended in Montana's largest city, Billings.

The balloon flew over the state of Montana, where 150 intercontinental ballistic missiles armed with nuclear warheads are stored at a military base in the north, according to the Wall Street Journal.

Similar incidents have happened in the past, however this time the device has remained over US territory longer than usual. According to the Pentagon, the balloon poses no danger to aircraft due to its high altitude.

The Pentagon also said that Washington saw the balloon as having limited added value for intelligence gathering.

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