Young people in America are exposed to an increasing amount of unwanted pornography online, a new study has revealed.

Research by the University of New Hampshire for the National Center for Missing and Exploited Children compared the experiences of young people online in 1999-2000 with those in 2005.

The results showed a marked increase in exposure to unwanted pornography among users aged 10-17, up to a third from 25 per cent previously. The rise was due to more aggressive tactics by pornography business and improved speed and capacity of computers and internet connections to handle images.

Cyber bullying is also on the increase, with almost one in 10 experiencing some form of online harassment compared to six per cent in the previous study.

The results showed a decrease in the number of sexual solicitations received by young users, down to one in seven compared to one in five previously.

Young people are also more cautious online, with fewer visiting chat rooms or speaking with people they didn't know.

The number of predators attempting to make physical contact with children, however, has not declined.

A large amount of the pornography young people encounter was found to be particularly graphic and much involved violence.

"Internet providers and host sites need to do more to protect youth from unwanted pornography. They need better filtering and blocking and more and easier ways for youth to report problems," said Janis Wolak, one of the authors of the study.