China regards Taiwan as part of its territory and has not ruled out the use of force to bring the self-governing island under its control.
Japan and China were involved in a war of words on Monday after Prime Minister Sanae Takaichi said a Chinese attack on Taiwan could constitute "a survival-threatening situation" for Japan requiring the use of force.
Takaichi, who was elected as the country’s first female prime minister last month, said on Friday that Chinese use of force around Taiwan would qualify as "an existential threat."
The latest diplomatic spat started on Friday when Takaichi was asked in parliament about scenarios that would constitute a threat to Japan.
The ultraconservative Takaichi cited the examples of a Chinese naval blockade on Taiwan and any military action to prevent the arrival of US forces.
"If it involves the use of warships and military actions, it could by all means become a survival-threatening situation" in which Japan could use force for defence, she said.
Her comments sparked a sharp rebuke from Beijing over the weekend.
"We have no choice but cut off that dirty neck that has been lunged at us without hesitation. Are you ready?" Chinese Consul General Xue Jian said in a message posted on X, which was later deleted.
Xue also criticised past remarks made by former Prime Minister Shinzo Abe and other Japanese lawmakers that a "Taiwan emergency is Japan emergency” as "blatant interference of Chinese internal affairs and violation of sovereignty" that require a retraction and apology.
On Monday, Japan's Chief Cabinet Secretary Minoru Kihara said Tokyo had protested to Beijing over Xue's X message.
"While the intent of (Xue's) post is not entirely clear, we must say the comment...was extremely inappropriate," Kihara said.
Japan’s Foreign Ministry has since lodged a "strong protest" and demanded the post deleted. Kihara urged China to provide an explanation.
The row suggests that Japan-China relations may not be smooth under Takaichi, despite what appeared to be a cordial meeting with Chinese leader Xi Jinping in South Korea during the recent Asia-Pacific Economic Cooperation summit.
Japan's new leader has committed to beefing up the military and met Taiwan's representative to the summit the day after her meeting with Xi, angering China.
In Beijing, Chinese Foreign Ministry spokesperson Lin Jian said Xue's post was a personal one "directed at the erroneous and dangerous remarks that attempt to separate Taiwan from China’s territory and advocate military intervention in the Taiwan Strait."
He blamed some Japanese politicians and media for hyping up the issue to confuse public perception and divert attention.
China has lodged "solemn complaints and protests" with Japan over Takaichi's remarks, Lin said.
"Is Japan attempting to challenge China's core interests and obstruct the great cause of China’s reunification?" he said. "Where does Japan intend to take China-Japan relations?"
China regards Taiwan as part of its territory and has not ruled out the use of force to bring the self-governing island under its control.