The Norwegian Nobel Committee, which awards the Nobel Peace Prize, says it is "deeply worried" about the situation of Liu Xia, the widow of Nobel laureate Liu Xiaobo, after his death on Thursday.
Friends of Ms Liu say they have been unable to contact her and that ensuring her freedom was now a top priority.
"The Norwegian Nobel Committee is deeply worried about Liu Xia's situation in the aftermath of her husband's tragic death," Olav Njoelstad, the secretary of the committee, said in an emailed statement.
Chinese authorities confirmed Mr Liu died from multiple organ failure after not being allowed to leave the country for treatment.
Rights groups and Western governments had urged China to allow Mr Liu, a renowned dissident, and his wife to leave the country to be treated abroad.
But Beijing had warned against interference in its internal affairs, saying Mr Liu was getting the best care possible and was being treated by renowned Chinese cancer experts.
But the Nobel Committee have now called on Chinese authorities to lift all restrictions placed on Liu Xia.
"If she wants to leave China, there is no justification for denying her the opportunity to do so," the committee said.
Ms Liu has been under effective house arrest since her husband won the Nobel Peace Prize in 2010 and was allowed to visit him in prison about once a month.
Hu Jia, a fellow dissident and family friend, said: "Now, we are most concerned about Liu Xia, but there has been no information about her."
"All the willpower and force we put behind freeing Liu Xiaobo, we have turned to Liu Xia," he said.
Efforts should also focus on Liu Hui, the younger brother of Liu Xia, who was sentenced to 11 years in prison in 2013 for fraud and to whom Liu Xia is very close, Hu added.
Several other friends said they were unable to reach Liu Xia, who suffers from depression, or confirm her whereabouts.
Nobel Peace chief says China declined visa application
Friends have also started calling to be allowed involvement in Mr Liu's funeral arrangements.
More than 150 friends and supporters have signed an open letter announcing plans for an "online memorial" to Mr Liu, urging authorities to release his body and allow a public funeral.
"We will pay close attention to how Liu Xiaobo's funeral will be arranged," Shanghai-based writer and a friend of the faamily, Wen Kejian, said.
"We, at the very least, hope to have the opportunity to go to Shenyang or Beijing to send him off."
The leader of the Norwegian Nobel Committee said the Chinese consulate in Oslo refused to receive her visa application for travel so she could attend the Mr Liu's funeral.
"I was told that my visa application was incorrectly filled in ... because I did not have an invitation from the person I was visiting," Berit Reiss-Andersen said.
"When I told them I would be attending a funeral and that the person had passed away, I was told I should try a relative.
"I told them she was kept in isolation ... I was also told that I should have a hotel and plane ticket booked."
Liu allowed on medical parole but never freed
Mr Liu, a prominent participant in the Tiananmen pro-democracy protests of 1989, was jailed for 11 years in 2009 for "inciting subversion of state power" after helping to write a petition known as Charter 08 calling for sweeping political reforms.
He was not able to attend the Nobel Peace Prize ceremony in 2010, which he was awarded for his "long and non-violent struggle to end one-party rule in China", and it led to major diplomatic issues between Norway, who host the Nobel Prize, and China.
Mr Liu was diagnosed with terminal liver cancer in May and had recently been moved from prison to a hospital in Shenyang for treatment.
Though allowed out on medical parole Mr Liu was never freed, spending his final days in the hospital surrounded by security guards.
Ms Liu was at the hospital as her husband's health deteriorated over the past couple of weeks.