A health scare was the likely reason for the unexplained absence of Xi Jinping, as secrecy surrounding the man set to become China's next leader fuelled intense speculation, experts said Tuesday.

Vice President Xi's disappearance from public view comes at a crucial time for China with just weeks to go before he is expected to be named as the next leader of the Communist party in a generational handover of power.

Authorities have maintained strict silence on his whereabouts, refusing to answer questions on why he has missed four scheduled meetings with foreign dignitaries in the last week, including the US Secretary of State and Denmark's prime minister.

The failure to provide any explanation for why Xi has not been seen in public since September 1 has given rise to rumours online and in overseas Chinese-language media, including that he had been involved in a car accident.

Some have speculated that it points to serious instability at the heart of the party, which has suffered two major political scandals this year in the downfall of former leader Bo Xilai and the reported death of a senior official's son in a high-speed Ferrari crash.

But experts said that a relatively minor illness or injury -- and the Communist party's unwillingness to reveal any vulnerability in its future leader -- was the likeliest explanation.

Willy Lam, a politics expert at the Chinese University of Hong Kong, said he had heard from informed sources that Xi had suffered a sporting injury serious enough to require hospitalisation, but not life-threatening.

"My information is that he is on the mend. It's just that they don't feel comfortable about showing him in public," said Lam, who added that the health of top officials was still considered a "closely guarded state secret" in China.

"It has never happened, at least since after the Cultural Revolution, for a top leader to just drop out of sight one month before the party Congress," he said.

"It shows the party, for all the promise about transparency, is still a black-box operation."

The foreign ministry refused Tuesday for the second day running to answer questions about Xi at its daily media briefing.

"I hope you can ask a serious question," said spokesman Hong Lei in response to a question about the rumours surrounding the vice-president.

Internet searches for Xi's name and for the Chinese term for "back pain" -- one of the ailments he is said to be suffering from -- were blocked Tuesday under China's vast online censorship system.

Users of Sina Weibo, a popular microblog similar to Twitter, relied on other terms like "crown prince" and "she", a homonym for Xi, to lament the lack of clarity.

Some speculated that a recent row with Japan over contested islands was meant to draw attention from the missing leader.

"The country's sensitive crown prince has had another incident and they are diverting the subject, that's all. After a few months Diaoyu will not be mentioned again," posted one.

China's state-run media have steered entirely clear of the subject, and the BBC's international news channel went off air briefly after mentioning Xi's name.

Xi, who also cancelled on Singaporean prime minister and a Russia official last week, was last seen in public giving a speech at a top Communist party school in Beijing on September 1.

Adding to the mystery, Danish prime minister Helle Thorning-Schmidt told AFP on Tuesday she had never planned to meet Xi -- even though the Chinese foreign ministry had invited foreign media to a photocall of their talks.

"The drama is raised because of the lack of transparency," said Scott Kennedy, the Beijing-based director of an Indiana University research centre on China.

"I think it could very well be a health issue. But if it was something life-threatening then I don't think the rest of the leadership would have gone about their usual business or travelled internationally.

"My guess is we'll see a picture of Xi Jinping sometime soon, hosting somebody, visiting a factory or hospital or something like that. That's my guess as to how they would address it -- rather than saying anything."