It is the first news of Mr Liu since his arrest in early December
A prominent Chinese dissident whose fate has been unknown since he was detained early last month is now reported to be under house arrest.
Liu Xiaobo is under "residential surveillance" at a secret location in Beijing, human rights groups say. The measure generally precedes a trial.
Mr Liu was detained the day after he and other dissidents launched Charter 08, a wide-ranging call for democracy.
He is a government critic and veteran of the Tiananmen Square movement.
Until now, nothing had been heard of the former professor since his detention by police on 8 December.
But fellow activist Jiang Qisheng told AFP news agency on Friday that Mr Liu was being held in pre-trial detention "somewhere in the suburbs of Beijing", adding: "I don't know exactly where."
Calls made to Mr Liu and his wife went unanswered on Friday.
A statement from Amnesty International condemned Mr Liu's detention and called on the authorities to make public any information about his alleged crimes, the charges against him and his current whereabouts.
In December, more than 150 writers and activists from around the world wrote an open letter to China demanding that Mr Liu be freed from jail.
The letter expressed "deep concern" over the ongoing "arbitrary detention" of Mr Liu.
In response, China's foreign ministry said the country was under the rule of law and opposed foreign interference.
Mr Liu was one of more than 300 prominent Chinese intellectuals to sign the online petition in December.
Released to coincide with the 60th anniversary of the Universal Declaration of Human Rights, the charter called for greater freedoms and democratic reforms in China, including an end to Communist one-party rule.