By Christian Shepherd and Philip Wen

BEIJING, July 14 (Reuters) - Friends of China's Nobel Peace Prize-winning dissident Liu Xiaobo, who died of liver cancer in custody, said on Friday they are still unable to contact his widow, Liu Xia, and that ensuring her freedom is now a top priority.

Liu Xiaobo, 61, was jailed for 11 years in 2009 for "inciting subversion of state power" after he helped write a petition known as "Charter 08" calling for sweeping political reforms.

Liu Xia 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. She suffers from depression.

Liu Xiaobo died on Thursday after suffering multiple organ failure. He was recently moved from jail to a hospital in the northeastern city of Shenyang to be treated for late-stage liver cancer.

Rights groups and Western governments have mourned Liu Xiaobo's death and also called for Chinese authorities to allow his wife and the rest of his family to move around freely.

United Nations High Commissioner for Human Rights Zeid Ra'ad Al Hussein also urged China to guarantee Liu Xia freedom of movement, and allow her to travel abroad should she want to.

China, however, said the case remained an internal matter.

"The handling of Liu Xiaobo's case belongs to China's domestic affairs, and foreign countries are in no position to make improper remarks," China's foreign ministry spokesman Geng Shuang said in a statement sent to Reuters on Friday.

"We call on relevant countries to respect China's judicial sovereignty and not to meddle in China's domestic affairs with this individual case," he said.

Hu Jia, a fellow dissident and family friend, said Liu Xia's freedom was now a top priority for Liu Xiaobo's supporters.

"Now, we are most concerned about Liu Xia, but there has been no information about her. She is at this moment the person who is suffering most," Hu said.

"All the willpower and force we put behind freeing Liu Xiaobo, we have turned to Liu Xia," he said, calling on the United States and Germany to continue pressuring China to free Liu Xia.

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 said.

Several other family friends also confirmed that they were still unable to contact Liu Xia or other members of Liu's family to confirm her whereabouts.

Shanghai-based writer Wen Kejian, another friend of the Liu family, said his supporters would "pay very close attention to Liu Xia's freedom".

"I think, no matter what, she should be granted complete and unconditional freedom," Wen said. (Reporting by Christian Shepherd and Philip Wen; Editing by Paul Tait)