China has taken its widening spat with Japan to the United Nations, accusing Tokyo of threatening "an armed intervention" over Taiwan and vowing to defend itself in its strongest language yet in the two-week-old dispute.

Japanese Prime Minister Sanae Takaichi committed "a grave violation of international law" and diplomatic norms when she said a Chinese attack on Taiwan could trigger a military response from Tokyo, China's UN Ambassador Fu Cong wrote in a letter on Friday to UN Secretary-General António Guterres.

"If Japan dares to attempt an armed intervention in the cross-Strait situation, it would be an act of aggression," Fu wrote, according to a statement from China's UN mission. "China will resolutely exercise its right of self-defence under the UN Charter and international law and firmly defend its sovereignty and territorial integrity."

Biggest bilateral crisis in years

Beijing views democratically governed Taiwan as its own territory and has not ruled out the use of force to take control of the island. Taiwan's government rejects Beijing's claims and says only the island's people can decide their future.

Japan's Foreign Ministry and Prime Minister's Office could not immediately be reached on Saturday for comment on Fu's letter, the strongest criticism of Takaichi yet from a senior Chinese official in the biggest bilateral crisis in years.

Takaichi, a conservative nationalist who took office last month, ditched the ambiguity that Japan and the US have long used regarding Taiwan when she told a questioner in parliament on November 7 that a hypothetical Chinese attack on Taiwan - which lies just over 100 km (60 miles) from Japanese territory - could be deemed "a situation threatening Japan's survival".

That is a legal designation that allows a Japanese prime minister to deploy the nation's military.

Takaichi's remarks sparked the tit-for-tat dispute with China that has spilled beyond diplomacy in recent days, with China saying it has "severely damaged" trade cooperation, while concerts of Japanese musicians in China have been abruptly cancelled.

Fu demanded that Japan "stop making provocations and crossing the line, and retract its erroneous remarks", which he said were "openly challenging China's core interests".

Ahead of this year's 80th anniversary of Japan's World War Two defeat, Beijing has increasingly invoked Tokyo's wartime atrocities and China's postwar role in setting up the UN as it criticises its Asian neighbour and seeks to reshape the international governance system.

China, a permanent member of the UN Security Council, has repeatedly emphasised two postwar declarations envisioning that Taiwan and other territories that had been occupied by Japan would be "restored" to Chinese rule.

The Potsdam and Cairo declarations form the basis for China's legal claims of sovereignty over Taiwan, though many governments view them as statements of intent, not legally binding accords.

Moreover, the declarations were signed by the Republic of China government, which fled to Taiwan in 1949 after losing a civil war with Mao Zedong’s communists. Taiwan held China's UN seat until 1971, when it was transferred to the Beijing government of the People's Republic of China.