The Chinese doctor who exposed Beijing's Sars cover-up last year is undergoing "brainwashing sessions" in custody, according to media reports.

Jiang Yanyong is being interrogated about his letter denouncing the 1989 Tiananmen Square crackdown, said sources quoted by the Washington Post.

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Mr Jiang - a doctor in China's People's Liberation Army - is being held under 24-hour supervision at an undisclosed location, the Washington Post said.

The authorities have threatened to keep him in custody until he "raises his level of understanding" about the crackdown on the student-led rallies for democracy on the Tiananmen Square, it said, quoting one of the sources familiar with the situation.

In response to the Washington Post questions, the Chinese government said in a statement: "Jiang Yanyong, as a soldier, recently violated the relevant discipline of the military."

"Based on relevant regulations, the military has been helping and educating him," the statement added.

Mr Jiang's family said they had not heard from him in more than a month, apart from a single handwritten note, the UK's Times newspaper reported.

"We are very concerned and have no idea when or if he will come back," a family member told the newspaper.