The United States said it was talking to people close to Chinese activist Chen Guangcheng to discuss how best to help the rights lawyer, unreachable since his release from jail two months ago.
Chen, who was jailed in 2006 after accusing family planning officials in the eastern province of Shandong of forcing women to have late-term abortions or be sterilised, has been under house arrest since he was freed in September.
Activists say he is under constant surveillance and remains cut off from the outside world.
"We are in contact with representatives of Mr Chen's family to discuss their views on how the embassy can best support Mr Chen," Richard Buangan, spokesman for the US embassy in Beijing, told AFP.
"The US government is concerned by reports that... Chen Guangcheng and his family members are currently prohibited from leaving their home, including for the purpose of seeking medical attention for serious health problems."
Late last month, a group of US lawmakers urged US ambassador Jon Huntsman to raise Chen's case with Chinese officials and to "send an officer to visit Mr Chen to check on his welfare and well-being."
But according to US-based activist group Human Rights in China (HRIC), no one -- not even Chen's mother -- has been allowed to visit the blind activist and his wife since early October.
The group quoted Yang Lin, another rights activist, as saying villagers had told him that authorities had mobilised more than 100 people to control the four entrances to the village and monitor Chen’s family around the clock.
"According to Yang, several security cameras have been installed inside and outside Chen’s home. At night, bright lights mounted outside their home, reaching their bedroom, stay lit," the group said.
HRIC executive director Sharon Hom expressed outrage at Chen's situation.
"To imprison a man in his own home who has already completed his prison sentence and is not subject to deprivation of political rights is an outrage," Hom said in a statement.
"Unfortunately, this is a reflection of the current regression of rule of law in China, even as the Chinese authorities assert that there is progress."
Chen, who has no formal legal qualifications, is what is known in China as a "barefoot," or self-taught, lawyer. He has been blind since childhood.
He helped people seek legal redress over a wide variety of injustices, with corrupt officials a particular target, and his current situation has caused concern.
"We urge the Chinese government to immediately restore the personal liberties, including freedom of movement, of Chen and his family," said the US embassy spokesman.
Local police refused to comment on Chen's situation when contacted by AFP.