A balloon floats over Columbia, Montana, on Friday. AP

China played down the cancellation of a visit by US Secretary of State Antony Blinken after a large Chinese balloon suspected of conducting surveillance on US military sites roiled diplomatic relations, saying that neither side had formally announced any such plan.

"In actuality, the US and China have never announced any visit, the US making any such announcement is their own business, and we respect that," China’s Ministry of Foreign Affairs said in a statement on Saturday.

Blinken was due to visit Beijing on Sunday for talks aimed at reducing US-China tensions, the first such high-profile trip after the countries' leaders met last November in Indonesia.

But the U.S. abruptly cancelled the trip after the discovery of the huge balloon despite China’s claim that it was merely a weather research "airship” that had blown off course.

The Pentagon rejected that out of hand - as well as China’s contention that the balloon was not being used for surveillance and had only limited navigational ability.

Uncensored reactions on the Chinese internet mirrored the official government stance that the US was hyping up the situation.

The balloon was spotted earlier over Montana, which is home to one of America’s three nuclear missile silo fields at Malmstrom Air Force Base, defense officials said.

President Joe Biden had declined to shoot down the balloon, following advice of defence officials who worried the debris could injure people below.

Meanwhile, people with binoculars and telephoto lenses tried to find the "spy balloon” in the sky as it headed southeastward over Kansas and Missouri at 18,300 metres.

The Pentagon also acknowledged reports of a second balloon flying over Latin America.

"We now assess it is another Chinese surveillance balloon,” Brig. Gen. Pat Ryder, Pentagon press secretary, said in a statement.

China's Ministry of Foreign Affairs did not immediately respond to a question about the second balloon.

On Saturday, China’s Ministry of Foreign Affairs again emphasised that the balloon's journey was out of its control and urged the US to not "smear” it based on the balloon.

Wang said China "has always strictly followed international law, we do not accept any groundless speculation and hype. Faced with unexpected situations, both parties need to keep calm, communicate in a timely manner, avoid misjudgments and manage differences.”

Blinken, who had been due to depart Washington for Beijing late Friday, said he had told senior Chinese diplomat Wang Yi in a phone call that sending the balloon over the US was "an irresponsible act and that (China’s) decision to take this action on the eve of my visit is detrimental to the substantive discussions that we were prepared to have.”

The first Chinese surveillance balloon that the Pentagon found flying over sensitive US ballistic missile sites may be guided by advanced artificial intelligence technology, a US expert said on Friday.

William Kim, a specialist in surveillance balloons at the Marathon Initiative think tank in Washington, told reporters that balloons are a valuable means of observation that are difficult to shoot down.

Shooting down a balloon is not as easy as it sounds, said Kim.

"These balloons use helium... It's not the Hindenburg, you can't just shoot it and then and then it goes up in flames."

"If you do punch holes in it, it's just kind of going to leak out very slowly."

Kim recalled that in 1998 the Canadian air force sent up F-18 fighter jets to try and shoot down a rogue weather balloon.

"They fired a thousand 20-millimeter cannon rounds into it. And it still took six days before it finally came down. These are not things that explode or pop when you shoot at them."

He said it was not clear if using surface-to-air missiles would work, because their guidance systems are designed to hit fast-moving missiles and aircraft.

Kim said the first Chinese balloon looked like a normal weather balloon but with distinct characteristics.

It has a quite large, visible "payload" -- the electronics for guidance and collecting information, powered by large solar panels.

And it appears to have advanced steering technologies that the US military hasn't yet put in the air.

Artificial intelligence has made it possible for a balloon, just by reading the changes in the air around it, to adjust its altitude to guide it where it wants to go, Kim said.

"Before you either had to have a tether... or you just send it up and it just goes wherever the wind takes it," he said.

"What's happened very recently with advances in AI is that you can have a balloon that... doesn't need its own motion system. Merely by adjusting the altitude it can control its direction."

That could also involve radio communications from its home base, he said.

But "if the point of it is to monitor (intercontinental ballistic missile) silos, which is one of the theories... you wouldn't necessarily need to tell it to adjust its location," he added.