Chinese activist Chen Guangcheng has left the US embassy to seek medical care and join his family, officials said Wednesday, as Beijing demanded a US apology on the eve of key talks between the two powers.

Chen, who riled Chinese authorities by exposing forced abortions and sterilisations under the "one-child" policy, fled house arrest on April 22 and sought refuge in the US embassy where he demanded assurances on his freedom.

Hours after Secretary of State Hillary Clinton arrived in China for long-planned meetings, the United States broke nearly a week of silence over his case and said that the dissident has been taken for treatment in Beijing.

"Chen Guangcheng has arrived at a medical facility in Beijing where he will receive medical treatment and be reunited with his family," a senior US official said on condition of anonymity.

The US official did not immediately provide more details on Chen, such as whether he would be allowed to return home or head to the United States. The 40-year-old self-taught lawyer, who has been blind since childhood, has voiced hope for staying in China.

Wednesday's nearly simultaneous announcements from the two countries may not have ended the row, with China demanding an apology for what it called interference in its affairs.

"China is very unhappy over this. The US action is an interference in China's internal affairs and China cannot accept it," Chinese foreign ministry spokesman Liu Weimin said, as quoted by the state Xinhua news agency.

"China demands that the US apologise and thoroughly investigate this incident, deal with the people who are responsible and ensure these types of incidents do not occur again," he said.

His flight came despite round-the-clock surveillance around his home in eastern Shandong province, where he has alleged that he and his family suffered severe beatings after he ended a four-year jail term in 2010.

In a video recorded after his daring escape and released online, he appealed to China's Premier Wen Jiabao to punish several local officials he said had made his family's life a misery.

Chen's case threatened to overshadow the annual meeting between leaders of the world's two largest economies on key issues ranging from North Korea's rocket launch to Syria.

Clinton has in the past repeatedly criticised China's treatment of the 40-year-old legal campaigner.

Before the Chen case, Washington had hoped to showcase small signs of progress in relations with China at the Strategic and Economic Dialogue, which also includes US Treasury Secretary Timothy Geithner.

Largely in response to inflationary pressure, China has let its yuan appreciate. Currency levels have long been a source of friction, with US lawmakers charging that Beijing keeps the value of the yuan artificially low to flood the world with cheap exports.

On other sore points, China has in recent weeks reduced imports of oil from Iran, spoken out -- albeit cautiously -- against a rocket launch by North Korea, and supported a peace plan for Syria after joining Russia in vetoing two UN resolutions.

South Sudan said China, a major oil importer, would lend the new nation $8 billion despite Beijing's longstanding ties to Khartoum. The US pointman on Sudan, Princeton Lyman, is joining Clinton to seek China's help in ending recent fighting.