Yahoo has begun testing a new anti-phishing service that the company says will help users from being tricked by fake Yahoo-affiliated websites.

The new service will let someone know if they have landed on a legitimate Yahoo sign-in Web page. The intention is to prevent them from entering their Yahoo ID and password on a site not recognised as such, as so likely to be a phishing site. The service only supports US websites at the moment and is not an official programme as yet.

Phishing has grown to become a major online security concern. Scammers set up legitimate-looking websites from well-known companies, such as banks, online stores and Web portals, and try to lure people to them via e-mail and other methods.

The idea is to trick people into entering sensitive information into the sites, such as passwords and credit card numbers, for malicious purposes, such as ID theft and fraud.

But the authorities are fighting back. Last week, a man from Miami was charged with setting up a number of phishing websites designed to steal credit card information from Hurricane Katrina relief donors.

Jovany Desir, 20, was indicted on five counts of wire fraud related to several scams, the US Attorney's office said. He is alleged to have set up fake websites designed to look like those of the American Red Cross, PNC Bank, eBay and Paypal. Two Canadian banks were also spoofed - Banque Nationale and Desjardins Credit Union.

The US Attorney's Office did not say how many victims were taken in by these most recent scams, but the Banque Nationale site received about 8,500 hits, it said.

The charges carry a maximum sentence of 50 years in prison and a fine of $1 million, although Desir made comparatively little money from his site, apparently selling them on for $150 each.