The US embassy in Beijing said it was "working to gather more information" about Liu's legal and medical status after authorities confirmed his transfer from prison to a hospital in northeast China.
With three years left to serve in his 11-year sentence, the 61-year-old democracy campaigner was diagnosed with terminal liver cancer on May 23 and granted medical parole days later, his lawyer Mo Shaoping said.
"We call on the Chinese authorities to not only release Mr Liu but also to allow his wife Ms Liu Xia out of house arrest," US embassy spokeswoman Mary Beth Polley told AFP.
Liu Xia, a poet, has been under house arrest since 2010, when her husband won the Nobel prize. She suffered a heart attack in 2014, when she was diagnosed with depression after years of detention, a rights group said at the time.
China should "provide them the protection and freedom such as freedom of movement and access to medical care of his choosing to which they are entitled under the Chinese constitution and legal system and international commitments," Polley said.
The Liaoning Prison Administrative Bureau confirmed Liu's parole Monday and said he was being treated by "eight renowned Chinese oncologists" at China Medical University No 1 Hospital in the northeast city of Shenyang.
"He has no special plans. He is just receiving medical treatment for his illness," Mo told AFP on Monday.
The writer was sentenced in 2009 for "subversion" after spearheading a bold petition for democratic reforms.
He was awarded the Nobel prize while in jail a year later and was represented by an empty chair at the ceremony in Oslo.
'Can't do chemotherapy'
International human rights groups and supporters demanded that Liu be granted the best medical care in China or abroad and criticised his treatment.
Su Yutong, a Chinese activist exiled in Germany, shared a video on Twitter which she said showed the campaigner's wife, Liu Xia, sobbing during a video call with a friend and saying her husband "can't have surgery, can't do chemotherapy".
Other friends of the couple told AFP they recognised Liu Xia in the footage.
The Nobel Committee said it was "delighted" to learn that Liu was finally freed, but that it "strongly regrets that it took serious illness before Chinese authorities were willing to release him from jail".
"Chinese authorities carry a heavy responsibility if Liu Xiaobo, because of his imprisonment, has been denied necessary medical treatment," it said.
Human Rights Watch and Amnesty International called on the authorities to ensure Liu's family can visit him.
"Adding injury to insult, Liu Xiaobo has been diagnosed with a grave illness in prison, where he should never have been put in the first place," Amnesty's China researcher Patrick Poon said.
Citing two other cases of critics who died in detention, Human Rights Watch's China director Sophie Richardson said the government "needs to be held to account for permitting yet another peaceful critic to fall gravely ill while unjustly detained."
Bold petition
Liu is one of only three people to have won the Nobel award while jailed by their own government.
China strongly condemned the prize as foreign interference in its internal affairs.
Diplomatic ties and trade talks between Beijing and Oslo were frozen, with China halting Norwegian salmon imports. Relations were only normalised last December.
Liu was arrested in 2008 after co-writing Charter 08, a petition posted online that called for the protection of basic human rights and reform of China's one-party Communist system.
Liu is also known for his efforts to help negotiate the safe exit from Tiananmen Square of thousands of student demonstrators on the night of June 3-4, 1989 when the military bloodily suppressed six weeks of protests in the heart of Beijing.
He was arrested immediately after the crackdown and released without charge in early 1991.
Liu was rearrested and served three years in a labour camp from 1996 for seeking the release of those jailed in the Tiananmen protests and for opposing the official verdict that their actions amounted to a counter-revolutionary rebellion.
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