WASHINGTON (Reuters) - The U.S. State Department said on Saturday that it was deeply concerned that a Swedish citizen and Hong Kong-based bookseller, Gui Minhai, had been detained in China and called for him to be allowed to leave the country.

The Swedish government has said that Gui, who has published books on the personal lives of President Xi Jinping and other Communist Party leaders, was taken into custody last week while traveling with Swedish diplomats to seek medical treatment in Beijing.

The European Union’s ambassador to China has called on the Chinese authorities to release Gui immediately, echoing demands from Stockholm.

“We are deeply concerned that Swedish citizen Gui Minhai was detained,” State Department Spokeswoman Heather Nauert said in a statement.

“We call on Chinese authorities to explain the reasons and legal basis for Mr. Gui’s arrest and detention, disclose his whereabouts, and allow him freedom of movement and the freedom to leave China,” she said.

The United States and European allies would continue to promote “greater respect for human rights in China,” she said.

Asked this week about the Swedish and EU demands for Gui’s release, a Chinese foreign ministry spokeswoman described the appeals as “baseless.”

Gui had been abducted in Thailand while on holiday in 2015, one of five Hong Kong booksellers who went missing that year and later appeared in custody on mainland China. The four others have returned to Hong Kong.

Hong Kong, a former British colony, returned to Chinese rule in 1997 with a guarantee of wide-ranging freedoms, including freedom of speech, but critics accuse Communist Party rulers in Beijing of creeping interference in the city’s affairs.

Chinese authorities said Gui was freed in October after serving a two-year sentence for a traffic-related crime in 2003.

Gui’s daughter Angela told Radio Sweden he was taken off a train by plainclothes police while en route to the capital to get medical attention for a neurological ailment.

Sweden’s Foreign Ministry has twice summoned China’s ambassador to Stockholm to explain the situation.