India on Tuesday protested to China over what it described as the “arbitrary detention” of an Indian citizen at Shanghai airport, after Chinese authorities reportedly refused to recognize her passport.

The woman, Pem Wang Thongdok from the northeastern state of Arunachal Pradesh, was stopped during a layover on her way to Japan on Nov. 21. She said she was held for 18 hours after officials claimed her passport was invalid, citing that her birthplace, Arunachal Pradesh, is considered by Beijing to be Chinese territory.

India maintains that Arunachal Pradesh is an integral part of its territory, while China calls it Zangnan, or Southern Tibet. The incident adds tension to relations already strained by border disputes, trade and technology restrictions, and competing regional interests.

China’s Foreign Ministry spokesperson Mao Ning said the case was handled “in accordance with laws and regulations” and claimed the woman’s rights were “fully protected,” denying that she was detained or harassed. Mao reiterated China’s position that it does not recognize Arunachal Pradesh as part of India.

India’s Ministry of External Affairs responded, reaffirming that Arunachal Pradesh is an “integral and inalienable part of India.” Spokesperson Randhir Jaiswal said Chinese authorities have not provided any valid explanation for the detention, calling it a violation of international air travel conventions and China’s own transit rules that allow visa-free passage for up to 24 hours.

The episode underscores how the dispute over Arunachal Pradesh continues to complicate efforts to improve ties between the two nuclear-armed neighbors.