China's government is responsible for the "enforced disappearance" of late Nobel laureate Liu Xiaobo's widow, her US-based lawyer has said in a formal complaint filed to the United Nations. Beijing faced a global backlash for its treatment of Liu Xiaobo when he died of liver cancer last month, making him the first Nobel Peace Prize laureate to die in custody since German pacifist Carl von Ossietzky in 1938, who had been held by the Nazis. His widow, poet Liu Xia, 56, was followed around the clock by security officials, and has not been in touch with anyone since about a day before her husband's death, her lawyer, Jared Genser, said in a statement to the UN Working Group on Enforced or Involuntary Disappearances. Liu Xia has been "held incommunicado in an unknown location by Chinese government authorities" since July 15, the day of her husband's funeral, the lawyer's statement read yesterday. "I demand that Chinese authorities immediately provide proof that Liu Xia is alive and allow her unhindered access to her family, friends, counsel, and the international community," said Genser in a separate statement emailed to AFP. He said international law defined "enforced disappearances" as situations where government officials are involved in depriving a person of her freedom against her will, and refuse to acknowledge that deprivation or conceal the disappeared person's fate - stating that all such conditions had been met in Liu Xia's case. Also watch The UN working group did not acknowledge to AFP receipt of Genser's complaint, but said that generally speaking, its process of issuing an "opinion" on the matter was a lengthy one that could take years. Beijing would be free to dismiss that non-binding outcome. The US, the European Union and the UN High Commissioner for Human Rights have called on Beijing to free Liu Xia, who had been under house arrest since her husband won the Nobel prize seven years ago - despite having never been charged of a crime. Chinese authorities have said she is a free citizen who was merely too grief-stricken by her husband's death to be in touch with any friends or counsel. A Chinese government spokesman Zhang Qingyang declined to disclose Liu Xia's whereabouts on July 15, telling the media only that it was "best for her not to receive too much outside interference during this period". "The relevant departments will protect Liu Xia's legal rights according to law," he added. Foreign journalists who have tried to visit the couple's Beijing home have been rebuffed and physically harassed. Seven people are currently detained by Chinese police for commemorating Liu Xiaobo, China Human Rights Defenders said yesterday. Authorities released photographs and a video of Liu Xia at her husband's funeral and also at a sea burial near the northeastern coastal city of Dalian. Liu Xiaobo was a veteran of the 1989 Tiananmen Square protests who was jailed in 2008 after co-writing a petition calling for democratic reform, and sentenced to 11 years in prison for "subversion" a year later.