A UN watchdog on Tuesday questioned Chinese officials over alleged widespread use of torture in the country's prisons, deaths in custody and crackdowns on activists and lawyers, decrying an atmosphere of "impunity".

"Torture in China really is a cause of concern, and impunity in China is the main cause," said one of the investigators, Abdoulaye Gaye, referring to information provided by a wide range of rights groups, including Amnesty International.

China's delegation insisted during the hearing at the United Nations in Geneva that it had made great strides in dealing with the problem, acknowledging though it was still "facing challenges".

The 10-member Committee Against Torture, which periodically reviews the records of the 156 countries that have ratified an international convention against the abusive practice, questioned the Chinese officials for nearly three hours on a wide range of concerns.

Allegations that torture is commonplace in detention, that suspects are held incommunicado, and some die in custody, with little to no accountability for the perpetrators, were among the issues raised.

George Tugushi, one of the committee's top investigators, questioned Chinese claims that all detainees have adequate access to medical care.

- Deaths in custody -

"Please explain the deaths that have occurred in Chinese detention facilities because persons with medical conditions were unable to receive medical attention on time," he said.

One such case was that of 52-year-old Chinese dissident Cao Shunli, who died in custody in March 2014 after allegedly being denied medical treatment for months.

Cao had been detained in 2013 shortly before she was due to travel to Geneva to attend a UN session on China's rights record.

Committee member Felice Gaer suggested Cao was not the last Chinese activist blocked from coming to Geneva.

She demanded explanations from the delegation about allegations that at least seven Chinese citizens who had planned to testify before the committee this week "have been threatened by Chinese authorities" and some had been detained.

The independent experts that make up the committee also questioned whether suspects had adequate access to lawyers, and voiced alarm over persecution targeting lawyers themselves.

According to a report by Amnesty last week, fewer than 20 percent of criminal suspects in China have access to lawyers, and some lawyers end up being tortured themselves while trying to protect their clients.

The Chinese delegation is set to respond the barrage of questions on Wednesday.

Wu Hailong, China's ambassador to the UN in Geneva and head of the delegation, meanwhile insisted in his opening remarks the country had made "major progress in our combat against torture."

He pointed to amendments to China's Criminal Procedure Law made in 2012 that made "illegally obtained evidence" inadmissable, and that since last year required that interrogations in all major cases are audio and video-taped.

Legislation also provided "more detailed regulations on the conditions and procedures of coercive measures, and strengthened the right to defence.

"All these serve as a systematic guarantee against torture at various stages of criminal procedures," he insisted.

He acknowledged though that "there is always room for improvement," adding: "We are clearly aware that China is still facing challenges in the prevention and prohibition of torture."