Pedestrians walk past an advertisement for the People's Liberation Army (PLA) on a screen near the Luohu border crossing in Shenzhen, China, on Thursday, Aug. 15, 2019.

Pedestrians walk past an advertisement for the People's Liberation Army (PLA) on a screen near the Luohu border crossing in Shenzhen, China, on Thursday, Aug. 15, 2019. Qilai Shen—Bloomberg/Getty Images

(Bloomberg) — The U.K. said it was “extremely concerned” by reports that a Hong Kong consulate worker was detained during a recent trip to mainland China, a case that threatens to add to strains between Beijing and London.

The statement came after news site HK01 reported that the U.K. consular employee, Simon Cheng, 28, was reported missing after failing to return from an Aug. 8 meeting in the adjacent city of Shenzhen. The U.K. Foreign & Commonwealth Office said it was providing the man’s family with support and seeking information from authorities in Hong Kong and Guangdong province.

“We are extremely concerned by reports that a member of our team has been detained returning to Hong Kong from Shenzhen,” the office said. Cheng is being held on the mainland, the Telegraph newspaper reported, citing the British Embassy in Beijing.

Chinese Foreign Ministry spokesman Geng Shuang told a regular briefing Tuesday in Beijing that he wasn’t familiar with the reports on Cheng’s disappearance. The Hong Kong police didn’t respond to requests for comment.

The incident comes at a sensitive time for the U.K. and China, which has accused the British government of meddling in its former colony by defending the rights of demonstrators. U.K. Foreign Secretary Dominic Raab spoke with Hong Kong Chief Executive Carrie Lam on Aug. 9 to discuss “concerns about the situation in the city and the protests there,” the foreign office said in a statement that made no mention of Cheng’s case.

Concerns about the safety of foreign diplomatic staff operating in China have increased since Michael Kovrig, a global security analyst on leave from the Canadian foreign service, was detained in December and later accused of espionage. The move came amid a spat between Beijing and Ottawa over the arrest of a Chinese executive accused of sanctions violation in the U.S.

Text messages published by HK01 suggest that Cheng and his Taiwanese girlfriend were concerned about his safety as he prepared to pass through immigration on his return home. “Passing through. Pray for me,” he told his girlfriend, identified only by her surname Lee.

China has stepped up border controls near Hong Kong as authorities seek to prevent the financial hub’s unrest from spreading. Travelers into China are being asked to unlock their smartphones so Chinese agents can examine their chats and social media, Bloomberg News reported last week, citing a person familiar the procedure.

It wasn’t immediately clear whether Cheng successfully crossed the mainland border and made it to the high-speed railway station staffed by mainland agents on the Hong Kong side. Also unknown was what, if any, diplomatic protections were available to Cheng, whose LinkedIn profile lists his job as a trade and investment officer with Scottish Development International.

Immigration authorities told Lee that Chen was being held in administrative detention, according to the HK01. Authorities can hold people for as long as 15 days under that process.

The Hong Kong Immigration Department told Bloomberg News that the agency has reached out the city’s trade office in Guangdong seeking information about the case. The Hong Kong Economic and Trade Office in Guangdong confirmed that it had received the request and said it would follow up with the agency.