The daughter of Hong Kong bookseller Gui Minhai has expressed "deep concerns" about the wellbeing of her father, saying his family has not heard from him even though it has been a week since his reported release from Chinese custody.

"This week I have slept with my phone on my pillow waiting for my father to call," Angela Gui said in a statement posted on social media on Tuesday. "I will continue to do so until he does."

In a related post on Twitter, she wrote that her family had been informed about the release of Gui - who is a naturalised Swedish citizen - but insisted that "he is NOT free".

Sofia Karlberg, a spokesperson for Sweden's foreign ministry, confirmed to Al Jazeera that her office received information about Gui's "release", and is "working to get more information" about his case.

Karlberg confirmed that the foreign ministry is in touch with Gui's family and the Chinese government, but added, "We don't go into the details" about the case.

In a statement posted on Twitter, Sweden's Foreign Minister Margot Wallstrom also said her office is "seeking further clarification" from China.

Mysterious disappearance

Gui had disappeared while on holiday in Thailand in October 2015.

He was one of five people to have gone missing that year from a Hong Kong bookstore and an affiliated publishing company, which specialises in political books critical of China's ruling Communist Party leaders.

Three months after his mysterious disappearance, Gui turned up in China claiming on state media that he voluntarily turned himself in to answer to a drunk-driving incident in 2003, which reportedly resulted in a death of a student.

Gui was later sentenced to two years in prison. The other individuals later reappeared in Hong Kong.

Gui's detention and the disappearances of the other individuals prompted fears that Chinese authorities have been using tactics that infringe on Hong Kong's legal system.

At that time, Angela Gui, who is based in Britain, told reporters she suspected her father was abducted, and that his detention was related to his work in Hong Kong.

'Strange phone call'

In her statement, Angela Gui said that after receiving the news of her father's impending release on October 17, Swedish embassy officials headed to the detention facility, where they had previously visited Gui on three different occasions.

"However, as they arrived in the morning of the 17th, an official told them that my father had already been released at midnight," she said.

"They were also told that he was 'free to travel' and that they had no idea where he was."

Then, on Monday, Sweden's consulate general in Shanghai reportedly received a "strange phone call" from someone claiming to be Gui, according to his daughter.

The caller reportedly informed Swedish officials that he would be applying for a passport, but that he needed to spend time with his mother, "who is ill".

"To my knowledge, my grandmother is not ill. My father is not in fact with her. It is very unclear where he is. I am deeply concerned for his wellbeing," Gui's daughter wrote.

Al Jazeera contacted the Chinese embassy in Sweden for comment but did not receive a response.

Meanwhile, China's Foreign Ministry said in a statement released to Reuters: "According to what we understand, because Gui Minhai has served his sentence for the crime of causing traffic accident casualties, he was released on Oct. 17".

Patrick Poon, China researcher at Amnesty International, said Gui's reported release was a "positive" development but added that it "must be treated with caution".

"It remains to be seen if he is genuinely free," he said.

From the time Gui went missing to his reappearance in China and the release of his televised confession, "authorities have shown a flagrant disregard for due process", Poon said.

"The system is designed to break people and force them to go along with the Chinese government's version of events if they are to have any chance of being released.

"If Gui Minhai is truly free, he must be able to leave China if he wishes and contact his family, free from any harassment by the Chinese government."