China has sentenced Swedish book publisher Gui Minhai to 10 years in prison on charges of illegally providing intelligence abroad and claimed him as a citizen, prompting Stockholm to call for his release in a case that has rattled diplomatic relations.

Gui, one of five Hong Kong-based booksellers known for publishing salacious titles about Chinese political leaders, was snatched by Chinese authorities while on a train to Beijing in February 2018, the second time he disappeared into Chinese custody.

The court in the eastern city of Ningbo said the Chinese-born Gui was convicted to ten years in prison Monday, and claimed that he voluntarily reinstated his Chinese citizenship in 2018. Since China does not recognise dual citizenship, "China, according to law, only recognizes him as a Chinese citizen," foreign ministry spokesman Zhao Lijian said Tuesday.

But Swedish foreign minister Ann Linde on Tuesday said in a statement to AFP that Stockholm continues to "demand access to our citizen to give him the consular support he has the right to." Sweden has "always been clear that we demand that Gui Minhai is released in order to reunite with his daughter and family. That demand remains," Linde said.

Gui first vanished in 2015 while on holiday in Thailand and eventually surfaced at an undisclosed location in China, confessing to involvement in a fatal traffic accident and smuggling illegal books. He served two years in prison but three months after his October 2017 release, he was again arrested while on a train to Beijing while travelling with Swedish diplomats.

His supporters and family have claimed his detention is part of a political repression campaign orchestrated by Chinese authorities. The Ningbo court also sentenced Gui to five years of "deprivation of political rights" -- which in practice means he cannot lead state-owned enterprises or hold positions in state organs.

Zhao said Gui's alleged crime "seriously damages China's national security and interests."

"China resolutely opposes any country, any organization, and anyone's using any form to interfere in China's internal affairs and judicial sovereignty," he said.

Rights prize

A video released by China three weeks after Gui's disappearance showed him purportedly confessing wrongdoing and blaming Sweden for "sensationalising" his case and "instigating" law-breaking behaviour.

Gui's friend, dissident poet Bei Ling, said at the time that Gui's confession was likely made under coercion. Chinese criminal suspects often appear in videotaped "confessions" that rights groups say sometimes bear the hallmarks of official arm-twisting.

The Ningbo court's decision to highlight Gui's citizenship was "obviously politically motivated" and intended to "stop the Swedish government from offering any support to Gui Minhai," Amnesty International researcher Patrick Poon told AFP.

Gui's detention has put a strain on relations between Sweden and China in recent years, with Stockholm repeatedly calling for his release. China called off two business delegation visits to Sweden after Culture Minister Amanda Lind defied a Chinese threat of "counter-measures" and presented a rights prize to Gui in November.

Sweden's former envoy to China Anna Lindstedt is being investigated for allegedly overstepping her duties and arranging an unauthorised meeting with Chinese businessmen and Gui's daughter to get Gui freed. Yang Hengjun, an Chinese-born Australian writer known for his series of spy novels, has also been detained in China since January 2019 and was recently charged with spying.

He reportedly once worked in China's foreign affairs ministry in Hainan province -- although this has been denied by Beijing.