Crimes linked to Internet dating sites more than doubled in the first half of this year, according to a National Police Agency report released Thursday.

Most victims were young females.

There were 793 cases from January through June, or about 2.6 times the number of similar crimes last year, the NPA said.

About 400 cases were crimes that violated the law banning purchasing sex from minors. The number of serious crimes, including rape, robbery and extortion, were also on the rise.

According to the report, 213 cases involved violations of ordinances protecting minors -- about 3.6 times the figure for the same period last year.

Females under the age of 20 made up 85.7 percent of the 692 victims. High school students accounted for 48.4 percent of that figure and junior high school students for 25 percent.

The number of rape cases rose to 23 from 20 last year, while that for indecent assault cases fell to three from seven.

The number of robberies increased to six from two, while that of thefts fell to 13 from 14 last year.

There was one murder, down from five a year ago.

The increase in the number of extortion cases was also notable, rising to 33 from eight last year. The number of cases involving threats and fraud rose to 13 each, both up from eight the year before.

Mobile phones were the most popular way of accessing the sites, and they were used in 95.6 percent of cases, the report says.