The wife of jailed Chinese Nobel laureate Liu Xiaobo appeared in public on Tuesday after years of house arrest to attend the trial of her brother, his lawyer said.

Liu Hui went on trial in Beijing on a fraud charge connected to a real-estate deal on Tuesday, his attorney Mo Shaoping said.

Liu Xia, who is Liu Hui's sister and Liu Xiaobo's wife, attended the hearing in what appeared to be her first trip outside her home in Beijing since being placed under house arrest after her husband was awarded the Nobel prize in 2011.

She has not been formally charged with any crime.

Liu Xiaobo was jailed for 11 years in 2009 for "subversion" after circulating a charter calling for democratic reforms.

A photograph obtained by AFP showed Liu Xia, 54, in a black car outside the court.

"I'm not free, but I'm pleased you are here," she said to press gathered outside the court, a diplomat present at the scene but who did not want to be named because of the trial's political sensitivity told AFP.

"(Liu Xia's) brother drove her to the court from home, we're eating together at the moment," Mo told AFP, adding: "Her mood is pretty weak."

He added that Liu Xia was scheduled to return home later that day. State security officials had probably followed her during her journey, he said.

Police detained Liu Hui in January on suspicion of committing fraud in connection with a real-estate deal, Mo told AFP. He had denied the charges in court, Mo said.

He added that the trial, held in Beijing's Huairou District court, lasted about four hours, and that a verdict would be announced soon.

Dramatic video footage emerged in December of a group of Chinese activists breaking through a security cordon to reach Liu Xia, who appeared frail and emotionally shaken by the visit.

The court refused to comment on the case on Tuesday.