Crimes linked to Internet dating services in Japan more than doubled in the six months to June, with teenage girls accounting for more than 70 percent of the victims, police said on Thursday.

The total figure came to 793 for the first half of this year, up from 302 in the same period last year, the National Police Agency said in an interim report on web crimes.

Cases of illegally paying for sex with minors including child pornography hit 410, surging from 133 a year earlier, a police spokesman said. Another 213 cases concerned sex with underage girls.

Some 73 percent of the victims of the crimes were girls aged between 13 and 18, police said, adding that 95.5 percent of the offences were committed in cases where mobile phones were used to access the Internet services.

During the period, police recorded one murder case in connection with Internet dating services, with the number of rapes totalling 23. Three crimes of extortion and six robbery cases were reported.

But the number of serious crimes linked to Internet dating services could be much higher as police admit they are facing difficulties in tracing illegal web-dating crimes due to the lack of regulation.

“Right now, it is difficult for us to crack down on illegal Internet dating crimes as there is no specific law on such services,” a police spokesman said.

Earlier this month, police arrested a Japanese man on suspicion of murdering a 16-year-old schoolgirl whose body, weighed down with concrete blocks was recovered from the sea. Police suspect the man had contacted the victim through an Internet dating service.

Jiji Press news agency said the police agency was considering how to deal with Internet dating services including the possibility of introducing new legislation to regulate them.

As of the end of February, Japan had more than 550 web sites on dating services and some 80 percent of people who access them use mobile telephones, police said.