China dissident Chen Guangcheng is in the US embassy in Beijing following his dramatic escape from house arrest, activist Hu Jia has told the BBC.
Mr Hu said Mr Chen - who is blind - had scaled a high wall and was driven hundreds of kilometres to Beijing.
Other, unconfirmed, reports say Mr Chen is under "US protection" while talks take place with Chinese officials.
Mr Chen escaped on Sunday, activists say, and has since released a video addressed to Prime Minister Wen Jiabao.
There are reports that his brother and nephew have now been held by police.
The rights group Human Rights in China quoted a source who knew about Mr Chen, and said his nephew Chen Kegui was taken away from his home by more than 30 police officers.
Mr Chen's escape complicates already tricky relations between China and the United States and could overshadow a visit to Beijing next week by US Secretary of State Hillary Clinton. She has repeatedly called for the dissident's release.
The US state department has refused to comment on the claim that Mr Chen is in its embassy. The US embassy in Beijing has also not commented.
In the latest development, the Texas-based ChinaAid group released a statement on Saturday saying it had "learned from a source close to the Chen Guangcheng situation that Chen is under US protection" in Beijing.
"High-level talks are currently under way between US and Chinese officials regarding Chen's status," said the group, which is led by Bob Fu, an American-based human rights campaigner and friend of Mr Chen.
UN human rights chief Navi Pillay said she was concerned for the wellbeing of Mr Chen and his family, who live in Dongshigu town, Shandong province.
"I'm disturbed to hear reports that other family members, including his brother Chen Guangfu and nephew Chen Kegui, have now been detained," she said in a statement.
Blogger Yaxue Cao says he spoke to Chen Kegui on Friday at about 01:30 local time (Thursday 17:30 GMT), and he has transcribed the interview on his blog Seeing Red in China.
The transcript suggests that at the time of the interview Chen Guangfu had already been detained.
Chen Kegui was awaiting police arrest, having initially resisted an attempt to detain him by unidentified men less than two hours before by slashing at them with kitchen knives.
"Around midnight, about two hours ago, they entered our property by jumping over the enclosed walls, they pried open the locks and kicked on the doors. I heard my mother crying inside, helplessly: 'Please don't come in! Please don't come in!'"
Chen Kegui, who often sobs during the interview, insists: "I did not take knives to go out to kill anyone. I was defending myself in my own home. They attempted to apprehend me without showing any warrant."
The interview ends with Mr Chen saying: "I don't know whether the police are coming. Perhaps they will send a sniper to kill me. They would accuse me of killing. It's all possible."
Chen Guangcheng, 40, was placed under house arrest after being released from a four-year jail sentence in 2010. Reports suggest authorities only realised he had escaped on Thursday.
Mr Hu - a friend of Mr Chen and himself a prominent activist and dissident - said he had met him in the last 72 hours, since his escape.
He said Mr Chen had fled to the US embassy in Beijing.
In his video addressed to Prime Minister Wen, delivered from a darkened room, Mr Chen said outwitting his guards had not been easy.
In the appeal, posted online by Boxun, a Chinese dissident news website based in the United States, he asks that:
- Prime Minister Wen investigate and prosecute local officials Mr Chen says beat up his family members
- The safety of his family be ensured
- Corruption in general in China be dealt with and punished according to the law
The Chinese authorities have come under international criticism for their treatment of him. At one point his daughter was barred from school. Many sympathisers who have tried to visit his home say they have been beaten up.
A self-schooled legal activist, Mr Chen is known for revealing rights abuses under China's one-child policy and has accused officials in Shandong province of forcing 7,000 women into abortions or sterilisations.
He Peirong, another China-based activist who had also campaigned for Chen Guangcheng, has also been detained at her home in Nanjing according to other activists.
The Chen affair comes at an unwelcome time for China's leaders, who have been embroiled in a lurid political scandal involving disgraced former party boss Bo Xilai.