China has taken its growing dispute 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 U.N. Ambassador Fu Cong wrote in a letter on Friday to U.N. Secretary-General Antonio Guterres.

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.