Nine men have been arrested on suspicion of making, publishing or possessing child pornography, police said on Thursday, with three of them accused of either sexually assaulting or secretly filming their younger sisters.

Officers believed the case involved some 190 videos and more than 730 photos containing child pornography.

They said those arrested, aged between 15 and 39, are suspected of downloading child pornography items from social media, websites, or peer-to-peer sharing software; while some had allegedly uploaded the files to cloud drives for storage.

Ngan Hoi-ian, acting superintendent of the cyber security and technology crime bureau, said two suspects are believed to have taken illicit photos and videos of their respective younger sisters, who were nine and ten-years-old at the time.

Another suspect, meanwhile, had allegedly sexually assaulted his younger sister, who is a minor, Ngan added.

"With regards to these three cases, we have worked closely with the Social Welfare Department to ensure the welfare of the children, and most importantly, to ensure that the children and the alleged abusers are not under the same roof," she said.

Michael Fung, a police clinical psychologist, said some offenders tend to rationalise their behaviour and convince themselves that they did not do any serious harm to children.

"But this is not the case... it has an ineffaceable impact on them," he stressed.