Britain confirmed Tuesday that one of five missing Hong Kong booksellers feared detained by Chinese authorities is a UK citizen, saying it was "deeply concerned" over the disappearances.
The case has sparked fury from lawmakers and activists in semi-autonomous Hong Kong, adding to growing unease that freedoms in the city are being eroded.
Lee Bo, 65, disappeared last week and was last seen in Hong Kong, where he is a resident.
All five missing men worked for the same Hong Kong-based publishing house Mighty Current, known for books critical of the Chinese government.
British Foreign Secretary Philip Hammond, visiting Beijing, told reporters: "We have urgently inquired with both Hong Kong and mainland authorities."
Hammond added that if Lee were charged with any offences, he should be tried in Hong Kong.
Foreign minister Wang Yi did not reply directly when asked whether China had detained the booksellers, but said policy towards Hong Kong remained "unchanged".
"We will continue to uphold the principles of 'one country, two systems', Hong Kong people administering Hong Kong, and a high degree of autonomy," he said.
He was also asked whether China would recognise Lee's British passport.
"Based on the basic law of Hong Kong and China's nationality law, this person in question is first and foremost a Chinese citizen," he said.
China does not recognise dual nationality of its own citizens.
An earlier statement from the Foreign and Commonwealth Office confirmed Lee was a British citizen, saying it was "deeply concerned by reports" about the disappearances.
The FCO urged the Hong Kong government to "honour its commitment" to press freedom.
It added that it hoped Chinese authorities would ensure the environment for media and publishers in Hong Kong supported "full and frank reporting".
- 'I feel unsafe' -
Hong Kong was handed back to Beijing by Britain in 1997 and enjoys freedoms unseen on the mainland. Chinese law enforcers have no right to operate in the city.
Police confirmed Tuesday Lee's wife, Sophie Choi, had retracted a report on her husband's disappearance, a move Amnesty said smacked of "intimidation".
"I believe he did it voluntarily, so I cancelled the report," Choi told reporters.
A friend of Lee who volunteers at the book store and did not want to be identified said: "His wife is on the brink of collapse."
He added he too was now afraid.
"I feel unsafe -- I don't know whether the next one will be me," he told reporters.
Amnesty said it was common for Chinese authorities to put pressure on those close to detainees.
"One wonders whether the same sort of intimidation is being used against associates and friends (of the publishers)," said Amnesty International's China researcher William Nee.
Rights groups also questioned the validity of a letter, published by Taiwan's Central News Agency, purportedly faxed to a colleague by Lee on Monday, saying he was well and had reached the mainland using his own means on an "urgent matter".
He said he was "assisting an investigation" but did not elaborate.
Lee's wife has previously said he called her from a number in the southern Chinese city of Shenzhen after he went missing.
Police are also probing the disappearances of three other missing employees who were Hong Kong residents.
One is a Swedish national, and embassies in Beijing and Bangkok are investigating his case.
In comments to a lawmakers Tuesday, pro-Beijing legislator Ng Leung-sing accused the five men of smuggling themselves to the mainland to visit prostitutes.
Ng said he had received the information in a message from "a good friend".