On Thursday, Beijing condemned US President Joe Biden's comments that Xi Jinping faced "huge problems", calling the words "extremely irresponsible".
The latest rhetorical skirmish between the United States and China comes after last week's downing of a Chinese balloon that Washington says was part of a spy fleet spanning five continents.
After a brief warming following the November G20 meeting between Biden and Xi Jinping, US-China relations have soured again, with Secretary of State Anthony Blinken canceling a visit to Beijing last week over the hot-air balloon standoff.
The U.S. says the high-altitude device that flew over at least one important U.S. military facility was intended for espionage.
China strongly denies the claims, saying it was a weather observation ship that went off course.
On Thursday, Beijing confirmed it had dropped an offer to hold talks between the defense chiefs of the two countries after Washington shot down the inflatable plane.
"This irresponsible and seriously mistaken approach by the US has not created a proper atmosphere for dialogue and exchange of views between the two militaries," the Chinese Defense Ministry said in a statement.
Earlier in the day, Washington said the balloon was part of a "fleet", adding that they had been spotted around the world for several years and urging allies to increase vigilance.
In an interview with PBS NewsHour that same day, Biden defended the decision to shoot him down and stressed that the US is not seeking conflict with China.
He also said that Xi has "huge problems," including an "economy that is not functioning very well."
“Can you think of any other world leader who would trade places with Xi Jinping? I can't think of any," Biden said.
China responded to the remarks on Thursday, when Foreign Ministry spokesman Mao Ning told a regular briefing that Beijing was "very displeased".
"Such rhetoric from the US is extremely irresponsible and goes against basic diplomatic etiquette," Mao said, adding that Beijing "strongly opposes it."
He also doubled down on his position that the balloon shot down last week was for civilian purposes, saying claims that it was part of the fleet were part of an "information war" against China.
"The US ignoring China's repeated explanations and messages, overreacting and abusing force are irresponsible," Mao said.
"The international community can see very clearly what the world's largest spying, monitoring and surveillance country is."
"Be vigilant"
But in the face of growing concern about Chinese espionage in the skies, Japan said on Thursday it was coordinating with Washington to analyze unidentified aerial objects seen over the country in recent years.
In June 2020, a mysterious balloon-like object was spotted over northern Japan, and local residents posted photos on social media.
At the time, authorities said they were baffled by the object, which in close-up photos from residents and the media appeared to be a balloon attached to crossed sticks with propellers.
During a visit to Washington this week, NATO Secretary General Jens Stoltenberg said the US downed device showed the alliance's need to defend itself.
"The Chinese balloon over the United States confirms a pattern of Chinese behavior where we see China invest heavily in new military capabilities in recent years," Stoltenberg said.
"We also observe the strengthening of Chinese intelligence activities in Europe. They're using satellites, they're using cyber and, as we've seen over the United States, they're also using balloons," he said.
"So we just have to be vigilant."