Outspoken Chinese government critic Hu Jia has been detained in the latest move by security forces against people linked to activist Chen Guangcheng, who is reportedly under US "protection".
Just hours after Hu had told AFP on Saturday he believed his blind friend Chen was safe in the US embassy in Beijing, his wife said on Twitter overnight that her husband had been taken away by the authorities.
Self-taught lawyer Chen fled house arrest on April 22 with the help of supporters from under the noses of dozens of guards, and subsequently recorded a video alleging abuses against him and his family.
China Aid, a group run by former Tiananmen Square democracy activist Bob Fu, said Saturday it had learned from a "source close to the Chen Guangcheng situation" that the activist was now "under US protection".
The sensitive situation threatens to become a major diplomatic entanglement, with the US-China relationship often testy and US Secretary of State Hillary Clinton and Treasury Secretary Timothy Geithner due in Beijing on Thursday.
Chief White House counterterrorism adviser John Brennan told Fox News Sunday: "We are working very closely with the individuals involved in this," although he refused to give details or specify who he was referring to.
President Barack Obama was looking to strike an "appropriate balance" between the US "commitment to human rights" and having to "continue to carry out our relationships with key countries overseas", Brennan said.
China Aid said in a statement Saturday "high-level talks are currently under way between US and Chinese officials regarding Chen's status," also calling on the United States to ensure the safety of the activist and his family.
Chen, 40, won worldwide acclaim for exposing forced sterilisations and late-term abortions under China's "one child" policy, and for using his legal knowledge to help people battle a range of other perceived injustices.
He and his family were put under round-the-clock house arrest after he completed a four-year jail sentence in September 2010. He has said he was being punished for defiantly continuing to speak out.
The detention of Hu, who said he had met Chen since his flight, was the latest in a series of responses by security authorities to the escape.
Human rights groups previously said Chen's brother Chen Guangfu and nephew Chen Kegui had been taken into police custody after a violent confrontation early Friday.
He Peirong, one of Chen's supporters who helped transport him to a safe location, was arrested at her home in the eastern city of Nanjing on Friday, Fu previously said.
Hu, who is well known for activism in support of human rights, people living with HIV/AIDS and the environment, spent three months in prison last year for "attempted subversion of power."
His wife Zeng Jinyan tweeted that "people from the police will come to collect anti-viral medicine. His (Hu's) detention has been extended to 24 hours. I asked where Hu Jia would sleep, they said on a chair."
It was unclear what the medicines were for.
Later Sunday, Zeng said state security officials had called her again, "demanding to see me today to speak to me".
AFP made calls to the police station at which it was believed Hu was being held but officers said they did not know his whereabouts.
Locals in Chen's village of Dongshigu in east China's Shandong province expressed disbelief that "the blind man" could have fled the intense security surrounding him.
"I have not heard anything about the blind man escaping, there is no way he could escape," a local farmer who regularly passes by the village told AFP.
Chen's flight came two months after Wang Lijun, former right-hand man of disgraced Chinese leader Bo Xilai, went to the US consulate in the southwestern city of Chengdu and reportedly sought asylum.
He was turned down, but the incident was highly embarrassing for Beijing and provoked a major political crisis just months before a once-in-a-decade handover of power in China.
The last Chinese dissident known to have been granted refuge at the US embassy was Fang Lizhi, a key figure in the pro-democracy movement who spent a year under US protection after publicly supporting the 1989 Tiananmen protests.