Britain on Thursday said it was trying to contact an employee of its Hong Kong consulate being held in China while "urgently" seeking further information from Beijing and local authorities.
The Foreign Office in London said both British officials and relatives had been unable to speak to the missing staffer, Simon Cheng, who disappeared after visiting mainland China on August 8.
A Chinese foreign ministry spokesman confirmed Wednesday he had been "placed in administrative detention for 15 days as punishment" by police in Shenzhen, a city on the China-Hong Kong border, for breaking a public security law.
The spokesman also said because Cheng is from Hong Kong, the issue is an internal matter.
"We continue to urgently seek further information about Simon's case," a spokesperson for the British Foreign Office said in a statement.
"Neither we nor Simon's family have been able to speak to him since his detention.
"That is our priority and we continue to raise Simon's case repeatedly in China, Hong Kong and London and have sought to make contact with Simon himself."
The incident comes as relations between Britain and China have become strained over what Beijing calls London's "interference" in pro-democracy protests that have wracked Hong Kong for three months.
The unrest was initially triggered by a controversial law that would allow extradition to the mainland, but has since broadened into a call for wider democratic reforms.
China promised to respect the freedoms in the semi-autonomous territory after its handover from Britain in 1997, including freedom of speech, unfettered access to the internet and an independent judiciary.
Chinese authorities have increased their inspections at the border since the protests, including checking the phones and devices of some passengers for photos of the demonstrations.
Cheng is believed to have travelled to Shenzhen for a one-day business meeting on August 8.
That night, he was in the process of returning via high-speed train and sent messages to his girlfriend as he was about to go through customs.
He has not been seen or heard from since.
Beijing has faced criticism in the past for detaining foreign nationals amid ongoing diplomatic spats.
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