BEIJING - China invited medical experts from the United States and Germany to help treat dissident Nobel laureate Liu Xiaobo for cancer, a local government announced on Wednesday, in a softening of its stance ahead of this week's G20 summit in Germany.Liu, 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.He was recently moved from jail to a hospital to be treated for late-stage liver cancer.The hospital, in the northeastern city of Shenyang, made the decision at the request of the family and in consultation with the doctors already treating him, the Shenyang Bureau of Justice said in a short statement on its website.It provided no other details. Officials who picked up the telephone at the hospital said they were unaware of the case.A source close to Liu's family said the invitation was a positive step that greatly increased transparency around Liu's illness and the chance that he would receive the best possible treatment available."The fact that they specially chose the US and Germany suggests that the authorities are considering allowing Liu to travel to one of these two countries," the source said, although there were still questions about how the doctors would be chosen and what access they would have.The US embassy in Beijing declined to comment. Newly appointed US Ambassador to China Terry Branstad said last week the United States would like to see Liu treated elsewhere for his cancer.The German embassy also declined to comment.NERVOUS CHINAThe move comes ahead of President Xi Jinping's attendance at a summit of the Group of 20 nations in the German city of Hamburg on Friday and Saturday, a high-profile event where Xi will seek to project Chinese leadership on issues such as climate change and free trade.Diplomatic sources in Beijing say China has been nervous that the issue over the Nobel Peace Prize winner could overshadow Xi's appearance.An open letter issued by a coalition of rights groups, including those representing Tibetans and Uighurs, on Wednesday urged G20 leaders to press China for the unconditional release of Liu and grant him freedom to travel wherever he wants."Liu Xiaobo's 2010 Nobel Peace Prize illuminated the human and political rights of the people under China's rule, and created a real sense of hope. We urge you not to let that sense of hope fade," it said.The government has said Liu is being given the best care possible and that he is being treated by renowned Chinese cancer experts.However, a growing number of Western politicians and international rights activists have expressed concern about the quality of Liu's treatment and have said he should be given the choice to leave China if that was the best option.