NEW DELHI, India: India has lodged a protest with China over what it called an "arbitrary detention" of an Indian citizen at the Shanghai airport after the traveler from a northeastern state said she was held for hours because Chinese authorities refused to recognize her passport.

Chinese authorities stopped the woman during a layover in Shanghai en route to Japan on November 21. She was told her passport was invalid because her birthplace, the Indian state of Arunachal Pradesh, is considered by China to be its territory of Zangnan, or Southern Tibet.

Pem Wang Thongdok said in a post on X that she was held at the airport for 18 hours.

The latest episode strains a relationship already under pressure by the military standoff along the Himalayan frontier, restrictions on trade and technology, and competing regional interests.

The two nuclear-armed rivals have been working to stabilize their ties following the deadly border clash of June 2020. The latest incident highlights how the dispute over Arunachal Pradesh remains a significant obstacle to improving relations between the Asian neighbors.

China's Foreign Ministry spokesperson Mao Ning said China's border inspection authorities handled the case "in accordance with laws and regulations."

Denying claims that the individual was detained or harassed, she said the authorities had "fully protected the legitimate rights and interests" of the person involved.

Mao also asserted that Zangnan was a Chinese territory and that Beijing never recognized the state of Arunachal Pradesh, which it claimed India had illegally established.

India's Foreign Ministry spokesperson Randhir Jaiswal stated, "No amount of denial by the Chinese side is going to change this indisputable reality."

He said Chinese authorities haven't been able to explain the woman's detention, which was in "violation of several conventions governing international air travel."

"The action by the Chinese authorities also violates their own regulations that allow visa-free transit up to 24 hours for nationals of all countries," Jaiswal said.