Sweden says it expects China to release a Swedish book publisher who was taken off a train by Chinese police four days ago while in the company of his country's diplomats.

Key points: Mr Gui taken off train by Chinese police in company of Swedish diplomats

Mr Gui taken off train by Chinese police in company of Swedish diplomats He is a Hong Kong publisher specialising in tales about high-level Chinese politics

He is a Hong Kong publisher specialising in tales about high-level Chinese politics He first disappeared in 2015 while in Thailand and turned up in police custody in China

Swedish Minister for Foreign Affairs Margot Wallstrom said in a statement on Tuesday that China has given no clear explanation for the detention of Gui Minhai.

Sweden has already summoned China's ambassador in the Scandinavian country over Mr Gui's detention.

"We take a very serious view of the detention on Saturday of Swedish citizen Gui Minhai, with no specific reason being given for the detention, which took place during an ongoing consular support mission," Ms Wallstrom said in her statement.

"We expect the immediate release of our fellow citizen, and that he be given the opportunity to meet Swedish diplomatic and medical staff."

The European Union later joined in the calls for his release, with the office of EU foreign policy chief Federica Mogherini saying it "fully supports the public statement and efforts of the Swedish Government" on Mr Gui's behalf.

"We expect the Chinese authorities to immediately release Mr Gui from detention, allow him to reunite with his family and to receive consular and medical support in line with his rights," it said in a statement.

Mr Gui had been running a Hong Kong publishing company specialising in gossipy tales about high-level Chinese politics when he disappeared from his Thai holiday home about two years ago.

He was believed to have been spirited away by Chinese security agents to mainland China, where he later turned up in police custody.

In a videotaped confession that supporters believe was coerced, Mr Gui stated that he had turned himself in to mainland authorities over a hit-and-run accident.

He was released into house arrest in October in the eastern city of Ningbo, living in what his daughter Angela called a police-managed apartment.

Angela Gui told Radio Sweden that her father was on a train with two Swedish diplomats on Saturday when a group of police officers seized him.

She said her father was travelling to Beijing to see a Swedish doctor after he was diagnosed with amyotrophic lateral sclerosis, a neurological disease that he developed while in custody.

Mr Gui's 2015 abduction reinforced rising fears that Beijing was chipping away at the rule of law in Hong Kong, a semi-autonomous Chinese city that is promised civil liberties such as freedom of speech until 2047.

The books Mr Gui and his colleagues sold at their Causeway Bay Bookshop were popular with visitors from mainland China, where such titles are banned.

AP