The 61-year-old dissident, who has late-stage liver cancer, is on dialysis and is at the resuscitation stage, the hospital said in a statement.
The Chinese government has rebuffed worldwide appeals to let Liu seek treatment overseas, saying he is getting the best possible care from top domestic doctors.
Friends also said security in the area around the First Hospital of China Medical University in Shenyang, northeastern Liaoning province, had been stepped up, while Liu's friends and family were under increased surveillance.
Liu has terminal liver cancer and the condition of his liver has deteriorated further, while bilirubin levels continued to increase, the Shenyang Health Centre was quoted as saying by Efe news.
German doctor Markus Buchler tells Liu Xia the Chinese physicians are "very committed" to her husband's treatment.
Ms Nauert added that Washington is anxious not only about the welfare of Mr Liu, 61, but of other dissidents held by China.
"We understand that the Chinese hospital treating Liu has invited United States and German medical experts to China for consultations", she said.
Human Rights Watch's China director Sophie Richardson called the leak "grotesque propaganda".
"The patient is in a critically ill condition, the hospital is doing all it can to save him, and his family members understand the situation and have given their signatures", it added, without elaborating.
Taiwan's President Tsai Ing-wen said on Twitter that the self-ruled island that China claims as its own, was closely following reports that Liu was in critical condition.
"We continue to call on the Chinese authorities to grant him full parole and to release his wife from house arrest, and provide them the protections and freedoms - such as freedom of movement and access to appropriate medical care - consistent with the Chinese constitution, legal system and global commitments".
Foreign ministry spokesman Geng Shuang repeated his standard answer earlier on Wednesday that other countries should respect the country's judicial sovereignty and "not interfere in China's internal affairs under the pretext of an individual case".
If he dies, Liu would become the first Nobel Peace Prize laureate to die in custody since German pacifist Carl von Ossietzky, who passed away in a hospital while held by the Nazis in 1938.
When he was convicted of inciting subversion in 2009, he told the court that he hoped he would be "the last victim of China's endless literary inquisitions and that from now on no one will be incriminated because of speech".
At the Nobel Peace Prize ceremony in Oslo in 2010, he was represented by an empty chair. Liu's friends have said he wanted to be treated in Germany - and for his wife to go with him - and was still pressing to leave the country for treatment, even though he may not have long to live.