The parents of a boy who died at a private children's hospital in Beijing last year want police to file a criminal charge against the doctor suspected of causing the 3-year-old's death.

The incident caught public attention as several unnatural deaths recently in Beijing have raised concern whether the government's supervisory role is effective.

The parents took the boy to Beijing Shouer Liqiao Children's Hospital in Shunyi district on Oct 23 for dental treatment. The parents said the boy died during the treatment but the hospital didn't give any explanation for the death.

The mother said she heard her son yelling during the treatment and wanted to enter the medical room to have a look but was refused entry by medical staff. A few minutes later she was told the boy had died.

A news report from Beijing Times said an autopsy found the boy had suffocated on a cotton ball.

The parents visited the criminal investigation division of the public security sub-bureau in Shunyi district on Monday and asked the police to file a criminal complaint against the doctor, based on the forensic report, but were turned down.

They were told the report is not sufficient for the police to file a criminal case because it cannot tell whether it was an accident or medical malpractice, according to the news report.

The district's public health department said it would look into the matter and act in accordance with laws and regulations.

A reporter from Beijing Times said the doctor is still working at the hospital.

The reporter quoted Wang Zhenghai, who is in charge of medical services at the hospital, as saying the hospital would allow the doctor to work unless she is found guilty.

But the hospital refused to comment on the matter on Tuesday.

The parents have sued the hospital for compensation in a civil case, which began in December and is underway.