Disappeared Hong Kong bookseller and Swedish national Gui Minhai has been kidnapped for a second time -- this time while in the company of Swedish diplomats, the Swedish foreign ministry said on .



Hong Kong-based Gui "disappeared" under murky circumstances from his holiday home in Pattaya, Thailand on , only to reappear in China "confessing" to a decade-old alleged drunk-driving offense.



One of five Hong Kong booksellers detained by Chinese police for selling "banned books" to customers across the internal border in mainland China, Gui was "released" by the Chinese authorities last October, but his U.K.-based daughter Angela Gui said he was still not free.



Instead, Gui had been placed under various forms of control and surveillance to his birthplace, Ningbo, and had been reunited with his wife Jennifer, who is a German national, the Independent Chinese PEN writers' group said at the time.



He was once more taken away by more than 10 plainclothes police officers while traveling from Ningbo to the Swedish embassy in Beijing by train in the company of two diplomats from the Swedish Consulate in Shanghai, Radio Sweden reported on .



Sweden's foreign ministry confirmed the story to the station.



"This incident will be handled with the utmost seriousness," foreign ministry official Patric Nilsson said.



"We have taken strong measures at the highest political level."



Angela Gui said she was "shocked but not surprised" by her father's second disappearance.



"I'm most worried about his health," she said. "I am very scared that this means his time in China will be drawn out even longer and I am very worried that I might not get to speak to him again for a very long time."



"I am very worried over how he is doing, considering he might not have so much time left," Gui added.



Illegal move

The New York Times reported on that mainland officials told Swedish diplomats that the bookseller was suspected of sharing secret information with them.



Amnesty International China researcher Patrick Poon said he was "surprised and angry" at the incident.



"I am very surprised and angry that he has been taken away once again, and worse, when he was in the company of diplomats," Poon said. "The Chinese government has acted recklessly."



But he said the incident was unlikely to have occurred without orders from the highest level.



"It's likely that the police were acting on orders from the highest echelons of leadership," he said.



Poon called on the government to clarify Gui's detention and current status. "If they don't, then 's incident really is illegal," he said.



The writers' group Pen Hong Kong also called for clarification of Gui's status.



"Pen Hong Kong is extremely concerned about Gui Minhai, the Hong Kong’s based publisher and bookseller reportedly snatched by police while on a train in China under the eyes of Swedish diplomats," the group said via its Twitter account.



"We call for his release and clarity over what has happened."



'Shocking development'

And Sophie Richardson, China director for the New York-based Human Rights Watch (HRW), said Gui's disappearance was "a shocking development."



"This is a shocking development, for Chinese police to do this to a foreign citizen, in the company of his country’s diplomats, and when Chinese authorities have themselves said he is ‘free’" Richardson said via Twitter.



"If diplomats talking to their own citizens in #China is now considered spying, this is a massive problem," she wrote.



And Amnesty International researcher William Nee said the group remains "very concerned" about Gui.



"The government should stop any extralegal measures taken against him, and ensure he receives the medical care that he needs," Nee wrote in a tweet on .



In , the U.K. accused Beijing of breaching the handover treaty by "involuntarily removing" Gui's colleague British national Lee Bo across the internal immigration border to mainland China.



There is no record of Lee leaving Hong Kong, suggesting that he was spirited across the internal immigration border by Chinese police, while colleagues Lui Por, Lam Wing-kei and Cheung Chi-ping were also detained in late 2015 after they crossed the border into China.



Lui, Cheung and Lam were later released after making televised "confessions" with a set of instructions from China's state security police: to reappear in Hong Kong, refute reports of their disappearance, and claim to be voluntarily helping police with their inquiries.



But Lam Wing-kei, who refused to to stick to that script and has since traveled to the democratic island of Taiwan, has told RFA that Gui is very unlikely to be allowed to leave China now.



Lam said the authorities are likely to use Gui's relatives still living in China as "hostages" to prevent him from speaking out even if he is allowed to leave the country.



Reported by Pan Jiaqing for RFA's Cantonese Service. Translated and edited by Luisetta Mudie.