China’s government is responsible for the “enforced disappearance” of Nobel laureate Liu Xiaobo’s widow, her lawyer said on Wednesday in a formal complaint. Beijing faced a global backlash for its treatment of Liu when he died of liver cancer last month.

Liu Xia (56), 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 U.N. 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.

“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 Mr. 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 Ms. Liu Xia's case.

The U.N. working group did not acknowledge to AFP receipt of Mr. 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 U.S., the European Union and the U.N. High Commissioner for Human Rights have called on Beijing to free Ms. 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 Ms. 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 on Wednesday.

Authorities released photographs and a video of Ms. 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.