People who use "hook-up" apps to meet for casual sex must ensure their partner is above the age of consent, the National Crime Agency is warning.

It follows a reported rise in the number of teenagers phoning the NSPCC's helpline with bad experiences after downloading apps like Grindr.

One man contacted by Newsbeat said he had been "groomed" by a 24-year-old after going online aged 13.

App designers say users must confirm they are of appropriate age - over 17.

There are several free "hook-up" apps which show the profiles of nearby users - including their photographs and often their sexual preferences.

These differ from traditional online dating sites because they are "geosocial" - they can pinpoint users to within a metre, rather than general areas like towns - meaning they have become popular with people looking for casual sex.

The maker of two leading services, Grindr, which is intended for gay men, and Blendr, aimed at straight people, claims around 201 million adults use its services. Others include Scruff, Jack'd and Bender.

Adult responsibility

Aaron, now 18, says he downloaded Grindr when he was 13, lying about his age when he registered. He said he was just looking for people to talk to about his situation.

These apps do have terms and conditions in place but they're simply not being enforced and as a result children as being put at risk of serious harm Claire Lilley Head of Child Safety Online, NSPCC

"I was struggling with my sexuality so having something on my phone I could easily download was one way of meeting people who were gay that I could relate to," he said.

But soon he began messaging a 24-year-old man in his area.

"He knew I was obviously emotionally vulnerable. We used to go out for meals, go to the cinema, play computer games and stuff like that. Looking back he was grooming me."

The friendship developed and Aaron had his first sexual experience with the man, who has since been prosecuted and put on the sex offenders' register.

Aaron said: "There needs to be something done so that when you do download it, you can prove the age you actually are."

The National Crime Agency's warning comes on Safer Internet Day. Jonathan Baggaley, head of education said: "Just because we've not received huge reports doesn't mean this isn't a major issue. Clearly the very dynamic of this kind of relationship means young people won't report it."

He warned: "The law is very clear: regardless of how the conversation is started, regardless of who thinks they're in the driving seat the adult is always the one who has the responsibility. A child should never be blamed."

'Diligent moderators'

The NSPCC said it did not have specific figures on "hook-up apps" but said it had seen a rise in calls about them. It said in 2012-13, 1,061 young people contacted its ChildLine to talk about online sexual abuse.

Claire Lilley from the NSPCC said: "These apps do have terms and conditions in place but they're simply not being enforced and as a result children are being put at risk of serious harm. We know this because of the huge increase in calls to ChildLine over the last year about issues such as online grooming."

Kevin Robinson from Barnardo's said these geosocial apps can be more accurate at pin-pointing locations than many young people imagine.

Two young people the charity worked with had downloaded one of the apps and sent out a message saying something like, "We're bored and looking for a laugh."

"However, that message was picked up by someone using a geo-location system. That person tracked where their messages were coming from and turned up on their street and was parked outside their bedroom window," he said.

"Without more stringent sign-up checks, regulation and control, we can't get enough information from the app designers to help us track abusers."

In a statement Blendr - who also own Grindr - said: "We do our best to ensure all users follow our strict terms-of-service policy that require users to be at a minimum age of 18.

"We have a diligent team of moderators focused on monitoring and ensuring users adhere to our terms-of-service guidelines."

The Department of Culture Media and Sport said terms of use and age restrictions were added voluntarily by app developers but they were not legally enforceable as they had no basis in law.

Radio 1 Newsbeat and BBC Radio 5 live will broadcast a special joint programme about online safety on 11 February between 12:30 and 13:00 GMT.

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