India said it had registered a strong protest with China over what it said was the arbitrary detention of an Indian citizen at Shanghai Airport.

Indian media reported that a UK-based woman holding an Indian passport was stopped by Chinese authorities during a layover in Shanghai on November 21 and told that her Indian passport was invalid as she was born in the eastern state of Arunachal Pradesh.

Beijing says Arunachal Pradesh, which it calls Zangnan, is part of China, a claim New Delhi has repeatedly dismissed.

The woman, Prema Wangjom Thongdok, said she was not allowed to board her onward flight to Japan and was held for 18 hours, according to Indian media.

“Chinese authorities have still not been able to explain their actions, which violate several conventions governing international air travel,” Indian foreign ministry spokesperson Randhir Jaiswal said in a statement late on Tuesday.

A Chinese foreign ministry spokesperson said on Tuesday that the checks had been carried out in accordance with laws and regulations.

Jaiswal said India had taken up the incident “very strongly” with China.

The Asian giants have been cautiously strengthening ties after four years of hostility against the backdrop of US President Donald Trump’s unpredictable foreign policy, staging a series of high-level bilateral visits.

Indian Prime Minister Narendra Modi visited China in August for the first time in seven years, where he and Chinese President Xi Jinping pledged that the two countries were partners, not rivals.

India and China share a 3,800 km (2,400 miles) border that is poorly demarcated and has been disputed since the 1950s.

Ties were ruptured after a 2020 clash in the Himalayas, in which 20 Indian and four Chinese soldiers died in hand-to-hand combat.