But as the diplomatic spat escalated, Japan's foreign ministry said it summoned Beijing's ambassador on Friday after a Chinese consul called to "cut off" Japanese Prime Minister Sanae Takaichi's neck in a now-removed online post.

Last week, Takaichi told parliament that armed attacks on Taiwan -- which China claims as part of its territory -- could warrant sending troops to the island under "collective self-defence".

If an emergency in Taiwan entails "battleships and the use of force, then that could constitute a situation threatening the survival (of Japan), any way you slice it", she said.

Beijing has not ruled out the use of force to seize control of Taiwan.

China's Vice Foreign Minister Sun Weidong summoned on Thursday the Japanese ambassador, Kenji Kanasugi, according to a statement published on Beijing's foreign ministry website.

It said Sun made "serious demarches over Japanese Prime Minister Sanae Takaichi's erroneous remarks regarding China".

"If anyone dares to interfere with China's unification cause in any form, China will surely strike back hard," the statement said.

Hours later, Tokyo said it summoned Beijing's ambassador, Wu Jianghao, "and strongly protested against the extremely inappropriate statements" made by Beijing's consul general in Osaka, Xue Jian.

The foreign ministry urged Wu to ensure "the Chinese side takes appropriate measures", it added.

Xue had threatened in a social media post to "cut off that dirty neck without a second of hesitation".

He did not name Takaichi but quoted a news article about her remarks.

Japanese Foreign Minister Toshimitsu Motegi said on Thursday that the now-deleted post was "highly inappropriate".

"We strongly urge the Chinese side to continue taking appropriate measures to ensure that this does not affect the broad direction of Japan-China relations," added Motegi, who was in Canada for a G7 meeting.

- 'Stability' -

Japan's Chief Cabinet Secretary Minoru Kihara said that his government's position on Taiwan remains unchanged and "is consistent with the 1972 Japan-China Joint Communique".

The 1972 communique normalised bilateral relations, with Japan acknowledging the "One China" policy recognised by many other countries including the United States.

"Peace and stability in the Taiwan Strait are important not only for Japan's security but also for the stability of the international community," Kihara told reporters.

Beijing's foreign ministry on Thursday said it would "by no means tolerate" Takaichi's Taiwan comments.

"The Japanese side must correct its wrongdoing at once and retract the unjustified remarks," spokesman Lin Jian told reporters at a press briefing.

In response, Takaichi told parliament Monday she had no intention of retracting her statement and insisted it was consistent with Tokyo's long-standing policy.

But she said she would refrain from referring to specific scenarios in the future.

Takaichi is an outspoken backer of Taiwan, advocating security ties with the self-ruled island.

Security legislation passed in 2015 allows Japan to exercise the right to "collective self-defence" under certain conditions including if there was a clear danger to the country's survival.