A supporter holds a poster outside of the British Consulate in Hong Kong during a rally in support of an employee of the consulate who was detained while returning from a trip to China. Photo: Vincent Yu/AP

Beijing - An employee of the British consulate in Hong Kong has received 15 days of administrative detention in the Chinese city of Shenzhen, the Foreign Ministry in Beijing said on Wednesday.

Foreign Ministry spokesman Geng Shuang said the person violated China's public security law but didn't go into details.

"Frankly speaking, this is not a diplomatic issue," Geng added.

The detainment comes as China-Britain ties are growing tense over pro-democracy protests in Hong Kong, a former British colony, over the past few months.

Britain expressed concern on Tuesday over reports that the consulate employee had gone missing in mainland China and may have been detained.

The man was identified by Hong Kong media as Simon Cheng Man-Kit, a trade and investment officer at the Scottish Development International section of the consulate. Cheng had visited mainland China on business and had been unaccounted for since August 8, the Hong Kong Free Press newspaper reported.

Cheng's Taiwanese girlfriend said his last message had been sent as he was approaching the border on a high-speed train, according to news outlet HK01.

Cheng is a permanent resident of Hong Kong and had studied in Taiwan and Britain before returning to the Asian financial hub. It was not known whether he holds a diplomatic passport and what documents he used to enter mainland China.

Hong Kong has seen a wave of protests, sparked on June 9 by opposition to a bill that would allow extraditions to China.

The city, which was returned to China in 1997, is guaranteed a degree of independence under the "one country, two systems" principle, but many are concerned that Beijing is encroaching on its freedoms.