The growing world of social media apps with big teenage audiences has made the situation even more difficult to police, they say.

"Identity assumption by child sex offenders is increasing quite steadily," Detective Inspector Jon Rouse, who runs task force Argos, a specialist branch responsible for tackling online child exploitation in Queensland, Australia, told us.

Detective Inspector Rouse led a recent investigation that led to a 42-year-old man, who allegedly posed as Canadian singer Justin Bieber on a number of social media platforms in order to gain indecent images of children, being charged with more than 900 child sex offences.

"The fact that so many children across the world could believe that they were talking to Justin Bieber, and that Justin Bieber would make them do the things that they did, is really quite concerning," he says, "I think a re-evaluation of the way we educate children about safe online behaviour is really needed."

One mother of an 8-year-old girl, who has asked to remain anonymous, told BBC Trending that her daughter had downloaded a popular social media app for just two days before she was approached by an account impersonating a celebrity.

"The first message was inviting you to enter a competition and to win it you get a five minute chat (with the celebrity)," she says.

"And then the second message that came up was along the lines of 'all you need to do is send me a photo of you naked or of your vagina.' And then all these messages flew across the screen.

"Then the third message said 'don't worry about it. All the girls are sending me these photos. Just do it. It'll be our secret'. And then the last message was 'do it now'."

The problem is found across the internet, from big platforms like Facebook, Twitter and Instagram to newer platforms which have large teenage audiences.

Detective Inspector Rouse of task force Argos raised concerns about Musical.ly, a social platform which launched in 2014. "Lots of child sex offenders are utilising Musical.ly to groom children. That's a very well-known international fact, believe it or not," he says.

The issue is not that safeguards on Musical.ly are particularly different when compared to other social platforms, but rather that there are a lot of young people using the app. It's been downloaded by more than 50 million people under the age of 21, with a sizeable number of them being under the age of 16.

Musical.ly told BBC Trending: "We take the safety of our users very seriously and we have zero tolerance for inappropriate, illegal, or predatory behaviour on our apps. We urge our users to report any inappropriate activity to us."

Chatter about this issue has been trending in the UK since February, when Gemma Styles, the sister of One Direction singer Harry Styles, alerted her followers to a fake Twitter account in her brother's name. She said the account was "preying on vulnerable girls".

The account @PrvtHarryStyles claimed to be a private page belonging to the One Direction star in which he could give advice to girls with mental health issues. It had over 10,000 followers.

A One Direction fan named Amy told BBC Trending that she had been the one to notify Gemma Styles to the account. Amy adds adds that she's seen dozens of fake accounts pretending to be various members of One Direction.

"They work as kind of a network a lot of times, where a person will make a fake account and then they'll make a fake Louis account and a fake Liam account and a fake Zayn account and all these fake accounts will talk to each other which kind of props up all of them," Amy says.

A number of people have posted about alleged inappropriate behaviour towards underage girls by the fake Harry Styles account.

We were unable to verify any of these allegations but we were contacted by someone saying they were a 19-year-old from the United States claiming that she sent naked pictures to this account, believing it was Harry Styles that she was talking to. She said she engaged with the account for about a month.

BBC Trending contacted Twitter who said they don't comment on individual accounts, but they do have strict rules around impersonation. That account has now been closed down.

There have also been allegations made by Amy, and others on Twitter, that the person who ran the @PrvtHarryStyles account also ran a fake Harry Styles Instagram account under the name vharrystyles. That account had been active up until this week had 138,000 followers.

Amy told BBC Trending that she tried to report the account to Instagram, but had difficulty doing so. According to Instagram's impersonation policies, only the person who is being impersonated can file a report.

BBC Trending raised concerns about the account to Instagram. The photo-sharing site has now shut the account down, saying that they removed it because it violated their impersonation policies.

A number of social media platforms like Facebook, Twitter and Instagram have verification for public figures - a little blue tick logo that is supposed to allow famous people to signal that social media account is theirs.

But Sharon Girling, a safeguarding consultant who was the co-founder of the UK's Child Exploitation and Online Protection Centre, says underage children may not be savvy if they have not been properly educated about the internet.

"When you're nine or 10 or 12 looking at these accounts, they seem to be genuine and so as a consequence it's the younger element that is getting fooled into believing that they are legitimate," she says.

"We don't let people drive a car until they're 17 because it's illegal to drive a car and we understand people have got to have an understanding of their responsibilities.

"Yet we give mobile phones and apps to children as young as five and six without parents having any understanding of what they're doing on them."