China's top leader-in-waiting Xi Jinping, who set off a storm of rumours after not showing up at scheduled public events for over a week, is nursing an ailment.
It is possibly a back injury suffered while swimming, according to sources.
Mr Xi, who is due to take over the presidency of the world's second-largest economy in March next year, has skipped several meetings with visiting foreign leaders and dignitaries over the past week.
These include meetings with US Secretary of State Hillary Clinton and the premiers of Singapore and Denmark.
China's government, however, has declined to spell out what is ailing Mr Xi, 59 - in keeping with decades of official secrecy over the health of senior leaders.
"Xi injured his back when he went for his daily swim," a source close to the Beijing leadership said after Mr Xi's absence from the public stage was first noticed last week.
At that stage, he had failed to keep scheduled meetings with Mrs Clinton and Singapore Prime Minister Lee Hsien Loong.
The source declined to give further details on the injury, including exactly when and where the incident took place.
Another source, citing people close to Mr Xi, said: "He's unwell, but it's not a big problem."
Both sources spoke on condition of anonymity.
Internet speculation
The lack of an official explanation has fuelled Internet speculation surrounding Mr Xi's condition, and brought some pointed questions at briefings by China's foreign ministry.
Ministry spokesman Hong Lei was asked whether Mr Xi is in good health.
Sticking to his government's usual reticence,Mr Hong said he had no information to give, and declined to answer further questions on the matter.
China's popular Twitter-like microblogging site Sina Weibo blocked searches for Mr Xi's name, as is common with top leaders, but users as usual found ways to skirt the restrictions, referring to Mr Xi as "the crown prince".
"What's up with the crown prince? He's vanished for the last 10 days or so and the whole world is wondering where he is," wrote one user.
Mr Xi had been scheduled to meet visiting Danish Prime Minister Helle Thorning-Schmidt yesterday or a photo opportunity, according to a media advisory that had been circulated on 5 September, but the event did not take place.
Mr Thorning-Schmidt said a meeting with Mr Xi had never been on her programme.
"I think there has been a slight misunderstanding," she said.
"I would, of course, very much like to see him (Xi) and I think I will see him at some stage, perhaps on my next visit to China, but he was never in my programme which we released on Friday, so there shouldn't be anything new in my not seeing him."
The foreign ministry's website shows Mr Xi's most recent public appearance was at a ceremony at the ruling Communist Party's Central Party School in Beijing on 1 September.
The rumours about Mr Xi come at a tempestuous time for Chinese politics.
The 18th Communist Party Congress, at which China's next top leaders including Mr Xi are likely to be unveiled, is set to take place in October.
Earlier this year, senior politician Bo Xilai, once seen as a candidate for the top rung of party leadership, was suspended from the Politburo and his wife convicted of murdering a British businessman.
Gu Kailai was given a suspended death sentence last month.
In another scandal this month, a senior ally of President Hu Jintao was demoted after sources said the ally's son was involved in a deadly crash involving a luxury sports car.
China has been engulfed in the past by rumours about senior leaders.
Last year, the official Xinhua news agency was forced to come out with a one-line statement in English denying former President Jiang Zemin had died, following feverish online speculation and a report from a Hong Kong television station.
