LONDON (Reuters) - Video cameras secretly placed in British hospitals to monitor parents have uncovered horrific cases of child abuse and torture. Parents and step-parents were captured on film sadistically kicking, hitting and trying to suffocate their babies -- footage which has cast doubts about the alarming number of cot deaths and led to calls for stricter child-protection laws. Doctors at North Staffordshire Hospital in northern England who conducted the controversial study announced the findings at a news conference ahead of publication in the American medical journal Pediatrics next month. They said they were shocked by the catalogue of abuse which showed how vulnerable children are and urged the United Nations to assure that all countries have child protection procedures. In addition to the physical suffering, the doctors also had evidence in some cases of continuous emotional abuse. Hospital ethics committees approved the use of video cameras, which revealed that some parents who were affectionate and caring in the presence of hospital staff were cruel and abusive when they were left alone with their children. Most of the children had been admitted to hospital after losing consciousness, turning blue, holding their breath or suffering other life-threatening events. None of the children in the study were killed but others in their families had died under suspicious circumstances. Twelve of the 41 siblings of the 39 children had previously died suddenly and unexpectedly. Eleven of those cases had been classed as cot deaths, or sudden infant death syndrome, but four parents later admitted they had suffocated eight of those siblings. Doctors suspect that between two to 10 percent of all cot deaths are due to suffocation by the parents but the cases are difficult to prove. It recommended sweeping changes to the approach to child protection and said programmes were rudimentary in some countries and non-existent in others.