Click fraud remains a growing problem for search engines and online advertisers, according to a study by US-based consultancy Click Forensics.

The research, based on data from 1,300 online marketers, said average click-fraud rates had risen to 14.1% in the second quarter, up from 13.7% in the first three months of the year.

Highly priced search terms (classified by the firm as those costing more than US$2 per click) were unsurprisingly targeted more than others, with an average fraud rate of 20.2%.

Click Forensics said the greatest percentage of click fraud, more than 88%, originated within the United States and Canada.

Outside North America, the greatest amount came from within India, which saw a 26% increase in the second quarter.

The firm also found that tier one search engines such as Google had the lowest click-fraud rate, although its average fraud rate of 12.8% increased from 12.1% in the previous quarter.

Tier two and tier three search providers had higher rates (20.3% and 27.1% respectively) but these had fallen from 21.3% and 29.8%in the three months.

The statistics come at a time when click fraud is coming under the spotlight as firms focus the returns they are getting from pay per click ads.

Market research firm Outsell recently estimated that click fraud cost advertisers US$800 million last year.