The European Union has joined Sweden in calling on China to immediately release a Swedish book publisher who was reportedly taken off a train in front of his country's diplomats by Chinese police four days ago.

The Chinese foreign ministry on Wednesday indicated Gui Minhai, the Hong Kong-based book publisher, and the Swedish diplomats who were with him may have been breaking Chinese law.

Gui was first abducted in 2015, one of five Hong Kong booksellers whose disappearances became a symbol of the extent to which China was willing to reinforce its hard line on squelching political dissent and a free press – despite international criticism.

TRT World'sBritt Clennett reports from Hong Kong.