The US secretary of state, John Kerry, has urged China to free the Nobel peace prize winner Liu Xiaobo and his wife, Liu Xia, five years after the writer was detained over his call for democratic reforms.

Kerry also expressed concern about Xu Zhiyong, a well-known rights lawyer and activist who was arrested this year. Police have recommended prosecuting Xu, who has campaigned for officials to declare their assets.

Liu was detained in December 2008 shortly before the publication of Charter 08, a document he co-authored that called for reforms. He was jailed for 11 years for subversion.

Liu Xia, a poet, has never been accused of a crime but has been under strict house arrest since shortly after the news that her husband had won the Nobel prize. Officials have never acknowledged that she is being held.

She is believed to have developed severe depression. Her friend Zeng Jinyan said last week that she had requested the right to consult a doctor freely, the right for her and her husband to read the correspondence they write to each other and the right to work and receive an income.

Liu Xia had been financially dependent on her brother until he was jailed for 11 years for fraud this year. Supporters of Liu Hui believe his case was further retribution against the family.

Kerry said in a statement on Monday: "We strongly urge Chinese authorities to release Liu Xiaobo, to end Liu Xia's house arrest, and to guarantee to Liu Xiaobo and his family members all internationally recognised human rights protections and freedoms.

"As the United States builds a constructive relationship with China, US leaders will continue to raise concerns related to respect for the rule of law, human rights, religious freedom, and democratic principles with their Chinese counterparts."

China rebuffed Kerry's call. The foreign ministry spokesman Hong Lei said at a regular news briefing: "China is a country with rule of law, and all are equal before the law. Nobody can be above the law. Liu Xiaobo and Xu Zhiyong are Chinese citizens who broke the law and have naturally been punished according to Chinese law."

Xu has yet to be charged with any offence, let alone tried or convicted.

Hong added: "China's 1.3 billion people have the best right to talk about the country's human rights. We hope that the US side can act in accordance with the broader perspective of bilateral ties and do more to increase mutual trust."