Police received more tips about illegal Internet sites in the first six months of this year but made far fewer arrests compared to the same period last year, data released on Thursday showed.

Reports about overseas websites doubled from a year earlier to nearly 10,000, but police made arrests or took other law-enforcement actions in 40 percent fewer cases — all domestic ones, the data showed.

"Cracking down (on overseas sites) is a challenge because the reach of our investigations doesn't extend overseas," a National Police Agency official said.

The Internet Hotline Center Japan, commissioned by the agency, received a total of 17,992 tips during the six-month period, up 28.4 percent. Tips about overseas sites nearly doubled to 9,796, while those about domestic sites fell 11.5 percent to 8,196.

It was the first time that tips about sites based outside Japan outstripped those in the country in the half-year period since data were first collected in the second half of 2006.

Based on the tips, police took law-enforcement actions in 377 cases — all concerning domestic sites.

The largest number of tips — 14,940 — was about pornographic photos or videos, followed by 1,452 about child porn, the agency said.

The police said they requested site operators and Internet service providers to remove contents in 3,848 cases, with deletions made in 3,626 cases.