Hong Kong (CNN) An employee of the British consulate in Hong Kong who has been missing for almost two weeks has been detained in mainland China, the country's foreign ministry confirmed Wednesday.

Simon Cheng, a 28-year-old who works as a trade and investment officer for the British Consulate General Hong Kong, has been put in administrative detention for 15 days, China's Ministry of Foreign Affairs spokesman Geng Shuang said Wednesday.

"I want to be clear: This employee is a Hong Kong citizen, not a UK citizen. So he is Chinese and this is a purely internal Chinese affair," Geng said.

Cheng was detained by the Shenzhen police for violating China's Security Administration Punishment Law , which covers a range of offenses deemed too minor to be crimes. The law is often used as a preliminary measure that allows police to investigate a suspect before deciding whether to prosecute.

According to Cheng's girlfriend Annie Li, Cheng traveled from Hong Kong to the border city of Shenzhen on the morning of August 8 in his capacity as a trade officer. Cheng is employed by the consulate's Scottish Development International investment agency, which promotes trade between Scotland and other countries, and is often required to go to mainland China for work.

Cheng messaged Li on the night of August 8, just before he was about to go cross the border into Hong Kong, Li told CNN. She has not heard from him since.

"Simon has signed a contract with the British government," she told CNN. "If he hadn't been given that assignment, he wouldn't have needed to go to Shenzhen. Britain must take the responsibility in rescuing Simon."

In a statement sent to CNN Wednesday, a spokesperson for the British Consulate-General in Hong Kong said legal assistance was available for employees arrested for reasons related to their employment.

"However, we need to establish the facts before we can decide what action is appropriate," the spokesperson said. "That's why we are urgently seeking further information from the Chinese authorities about why Simon Cheng was detained. We remain in close contact with his family."

In a statement Tuesday, the British Consulate-General said it was "extremely concerned" by reports that a member of its team had been detained returning to Hong Kong.

On August 8, Cheng messaged Li to say he was on the high-speed train. He later messaged to say he was about to pass through the border, according to screenshots seen by CNN. If Cheng was on the high-speed train between Shenzhen and Hong Kong, the only immigration checkpoint is in the West Kowloon station . In parts of the station, Chinese laws apply despite it being in Hong Kong, which operates under a separate legal system from the mainland.

Li said Cheng is a Hong Kong permanent resident who was traveling on his mainland travel permit. She said he also has a British National Overseas (BNO) passport , a special document for people from Hong Kong, a former British colony, which entitles them to consular assistance from the UK but is not equivalent to UK citizenship.

China does not recognize dual nationality for any Chinese national, meaning it does not recognize BNO passports.

Simon has signed a contract with the British government. If he hadn't been given that assignment, he wouldn't have needed to go to Shenzhen. Britain must take the responsibility in rescuing Simon. Annie Li, Simon Cheng's girlfriend

Cheng's disappearance comes as Hong Kong approaches its 12th weekend of mass pro-democracy protests. Travelers told CNN that security at the border between Hong Kong and China had increased since the demonstrations began in June.

A Hong Kong man who recently crossed back to the city from China told CNN that border police appeared to be picking people at random to be subjected to extra searches. The man -- who asked not to be named -- said police scrolled through the photos on his phone.

On Wednesday, around a dozen people protested outside the UK consulate in Hong Kong, expressing concern over Cheng's detention and asking the UK to do more to assist him. "Today could be Simon, tomorrow could be me," said a 29-year-old at the protest.

In a statement posted online Wednesday morning, Cheng's family reiterated that Cheng had been in Shenzhen on a "business trip."

On August 9, the family contacted Hong Kong Police and the Immigration Department, and were told they could travel to mainland China themselves to report the missing person case to authorities there, the statement said . On August 10, the Immigration Department told the family Cheng had been "administratively detained," but that they had no information on why, where, and for how long he would be detained.

The family said they had not received any notice of administrative detention, which is supposed to be sent out within 24 hours of a person's detention. "We simply have never received any documentation confirming that Simon has been formally detained by the authorities," the statement said.

"We feel very helpless, and are worried sick about Simon," the family said . "We hope that Simon can return to Hong Kong as soon as possible."

In the statement, the family said that a lawyer representing them had gone to three detention centers in Shenzhen, but was unable to find Cheng. Police stations in Shenzhen and Guangzhou, and the Guangzhou Railway Public Security Bureau, also had no information on his whereabouts.

Authorities in both Hong Kong and mainland China -- which operates under a separate legal system from Hong Kong -- have released few details of the case. On Tuesday, Hong Kong Police Senior Superintendent Kong Wing-cheung confirmed that they had received a missing person report on August 9. The Hong Kong Immigration Department also confirmed it had received a request for assistance from Cheng's family and was providing them with advice and assistance.

CNN has reached out to the Guangdong Provincial Public Security Bureau and the Shenzhen City Public Security Bureau.