The latest rhetorical skirmish between the United States and China came after last week's downing of a Chinese balloon that Washington said was part of a spy fleet spanning five continents.

Following a brief warming after a November G20 meeting between Biden and Xi, US-China relations have once again nosedived, with Secretary of State Antony Blinken last week calling off a visit to Beijing over the balloon fracas.

The US has alleged the high-altitude device -- which crossed directly over at least one sensitive US military site -- was intended for espionage.

China has angrily denied the claims, arguing it was a weather observation craft that blew off course.

On Thursday, Beijing confirmed it had refused the offer of a call between the two countries' defence chiefs after Washington shot down the inflatable aircraft.

"This irresponsible and seriously mistaken approach by the US did not create a proper atmosphere for dialogue and exchanges between the two militaries," China's defence ministry said in a statement.

The previous day, Washington said the balloon was part of a "fleet", adding they had been spotted around the world for several years and urged allies to step up vigilance.

In an interview with PBS NewsHour the same day, Biden defended the decision to shoot it down and stressed that the US is not looking for conflict with China.

He also said Xi had "enormous problems", including "an economy that is not functioning very well".

"Can you think of any other world leader who'd trade places with Xi Jinping? I can't think of one," Biden said.

China hit back at the remarks Thursday, with foreign ministry spokesperson Mao Ning telling a regular briefing that Beijing was "strongly dissatisfied".

"This type of rhetoric from the US is extremely irresponsible and runs counter to basic diplomatic etiquette," Mao said, adding that Beijing "firmly opposed this".

It also doubled down on its position that the balloon shot down last week was for civilian purposes, saying claims it was part of a fleet were part of an "information war" against China.

"The US disregard for China's repeated explanations and communications, excessive reaction, and misuse of force are irresponsible," Mao said.

"The international community can see very clearly what the world's biggest country of espionage, monitoring and surveillance is."

- 'Be vigilant' -

But in the face of mounting concern over Chinese espionage in the skies, Japan said Thursday it was coordinating with Washington as it analysed unidentified aerial objects spotted over the country in past years.

A mysterious balloon-like object was seen over northern Japan in June 2020, with locals posting pictures on social media.

Authorities said then that they were baffled by the object, which in close-up images by residents and media appeared to be composed of a balloon attached to crossed sticks with propellers.

In a visit to Washington this week, NATO Secretary-General Jens Stoltenberg said the device shot down by the US showed the need for countries across the alliance to protect themselves.

"The Chinese balloon over the United States confirms a pattern of Chinese behaviour where we see that China over the last years has invested heavily in new military capabilities," Stoltenberg said.

"We've also seen increased Chinese intelligence activities in Europe. They use satellites, they use cyber and, as we've seen over the United States, also balloons," he said.

"So we just have to be vigilant."