The Chinese government's efforts to restrict access to certain websites, a phenomenon that has become known as the Great Firewall of China, is becoming something of a game of cat and mouse.

Over recent days mirror sites have gone up around the web in an attempt by the internet community to offer Chinese users a back door to such websites as Google and AltaVista.

Search engine AltaVista today slammed the blocking of its website in China and said that it was working on alternative methods of serving Chinese users.

"We were very concerned to learn that AltaVista is inaccessible in China, and contacted the Chinese government in an effort to determine whether or not they intentionally blocked our sites, and if so, why," said Jim Barnett, president and chief executive of AltaVista.

"In addition, we have been working on alternative ways to serve our Chinese users with additional URLs not in the AltaVista.com domain," he said.

But some of the URLs Barnett listed, such as Raging.com and AltaVista.ca, have already been locked out by China.

Google, meanwhile, is still accessible behind China's firewall through a mirror site - in the literal sense of the word. A site called elgooG, available here, offers all the resources of Google in reverse. Users even have to enter their queries in reverse, so a mirror is handy when using the site to be able to read the text.

Inside China, however, it has been reported that the government has tinkered with the country's DNS system and is redirecting requests for Google.com to TianWang Search, a search engine run by Beijing University. Although it goes against internet guidelines, China is not bound by any contract or legislation and can alter DNS information as it pleases.

Altavista's Barnett has been far from impressed by the actions of the Chinese government. "Free access to information is the cornerstone of our mission to provide access to information to the global community," he said.