US lawmakers and Chinese friends of author Liu Xiaobo on Wednesday appealed for his freedom, vowing not to forget the world's only jailed Nobel laureate two years after he won the prize.

Lawmakers voiced regret that little had changed in the past two years during a hearing on Liu Xiaobo at the Congressional-Executive Commission on China, which is tasked with examining Beijing's human rights record.

"We have not forgotten Liu Xiaobo and his wife. We commit to seeking their release from confinement and detention. We will not forget them next year, or the year thereafter -- regardless of the circumstances," said Representative Chris Smith, co-chairman of the commission.

Liu was arrested and sentenced to 11 years in prison for subversion after spearheading Charter 08, a bold petition for the protection of human rights in the one-party communist nation.

China's authorities tried to block out news of his Nobel Peace Prize in 2010 and put his wife, Liu Xia, under house arrest.

Testifying before the commission, Liu Min -- whose husband Yu Jie wrote a book critical of China's leadership and was working on a biography of Liu Xiaobo -- urged President Barack Obama to keep up pressure.

"I hope the American government, especially President Obama, who is also a Nobel Peace Prize recipient, can personally and directly put out a strong call to the Chinese government and demand that they release Liu Xiaobo and Liu Xia immediately," she said.

A friend of the Nobel laureate for more than a decade, she said that she lost her job and Chinese authorities put her and her husband under house arrest after finding her phone number on a note written by Liu Xia.

On December 9 last year, Yu Jie was "kidnapped by the police" and "was beaten and tortured and almost died," the wife told the commission.

Liu Min and her husband fled to the United States in January. Yu Jie defiantly published "Wen Jiabao: China's Best Actor," a critical look at the country's premier, in Hong Kong in 2010.

Last week, more than 40 high-profile Chinese writers based in the mainland sent an open letter to China's leader-in-waiting Xi Jinping urging him to free Liu Xiaobo.

The signatories included outspoken legal scholar He Weifang, human rights lawyer Pu Zhiqiang and AIDS activist Hu Jia.

A group of 134 Nobel prize winners from across six disciplines also signed a letter calling for Liu's release.