LONDON (AP) – Child pornography on the Internet is becoming more brutal and graphic, and the number of images depicting violent abuse has risen fourfold since 2003, according to an Internet watchdog report published Tuesday.

The British-based Internet Watch Foundation said in its annual review that it received nearly 32,000 reports of potentially illegal content on its hot line last year, marking a 34 percent increase from the previous year.

“Although there is a volume issue, the worrying issue is the severity and the gravity of the images is increasing,” said the foundation’s chief executive, Peter Robbins. “We’re talking about prepubescent children being raped.”

The foundation identified about 10,700 individual Internet addresses on 3,000 sites containing child pornography content.

About 80 percent of the children in the abusive images are female, and 91 percent appear to be children under the age of 12, it said.

More than three in five child pornography Web sites were hosted in the U.S., while nearly a third were based in Russia.

The U.S. National Center for Missing and Exploited Children last year completed a pilot program in partnership with financial institutions aimed at cutting off child pornography Web sites’ revenues. They identified sites that accepted credit card payments, then contacted individual banks to stop the flow of money to the sites. The center also operates a child sexual abuse hot line.

More than 20 such tip lines exist around the world, but the international scope of the Internet makes prosecution difficult.

Some commercial child abuse Web sites store their images on different servers and occasionally in fragments, with each piece stored in a different country. Individual fragments often are not illegal.

“Many of them hop around between countries, between legal jurisdictions, which is a problem of an international nature,” Robbins said Monday, before the report’s release. “Law enforcement agencies have to reconsider their strategy.”

One site has been reported 54 times since 2000 and has been found using seven different servers in various countries, the report said.

The foundation, which is funded by the European Union and the online industry, shares its information with law enforcement organizations, including Interpol. It also provides a list of offensive Web sites to British Internet service providers for them to block.

“You need many other bits in the jigsaw to make it work, but the prevention of access to these sites is one part of the solution,” Robbins said.