The United States on Tuesday urged China to release anti-corruption and free speech activist Yang Maodong as his wife voiced fear that he was being mistreated following his latest detention.

Yang, also known by his pen-name Guo Feixiong, was taken away on August 8 after a public talk in the southern city of Guangzhou in which he defended freedom of speech and the rights of the outspoken Southern Weekly newspaper, supporters said.

The United States is "deeply concerned" by Yang's detention and has raised concerns with China, said Jason Rebholz, spokesman for the State Department's bureau of East Asian and Pacific affairs.

"We call on Chinese authorities to release Yang immediately, cease any restrictions on his freedom of movement and guarantee him the protections and freedoms to which he is entitled under China's own laws and international human rights commitments," he said.

The call came as Yang's wife and daughter, who sneaked out of China to the United States in 2009, appeared at a hearing in the US Congress to voice concern about the activist.

Zhang Qing, Yang's wife, said that authorities have denied all access to her husband including to a lawyer, who tried to see him seven times.

"From previous experience, I know that he has been tortured every time he was in jail, so I think that he is being tortured right now as well," she said.

She said that she did not know where her husband was jailed but that he has previously gone on hunger strikes while in detention.

Yang has been detained four times since 2011, when he ended a five-year prison sentence for "running an illegal business."

Yang, who became well-known for highlighting corruption by a local Communist Party boss, was detained the latest time on charges of organizing a "mob," she said.

His daughter Yang Tianjiao, who lives in Texas with the nickname Sara, said that she has not seen her father since 2006, when she was 10, and had hung up a watercolor portrait of him that she painted.

"My dad is a great man. He is my hero," she told a House Foreign Affairs subcommittee.

"I, his daughter, always want my dad to have the freedom that he works so hard to achieve for others."

Rights groups have reported a rise in detentions of political activists since President Xi Jinping took office in March.