China's largest search engine, Baidu.com, confirmed Tuesday its website had been temporarily paralyzed after coming under cyber-attack, and an expert on network security warned major websites of domain name server (DNS) protection against hackers.

Baidu.com resumed operation at 11:30 a.m. after being down for three and a half hours. The company said later in a statement that Baidu's DNS in the United States was illegally attacked, without giving more information.

Wang Zhantao, an expert with Beijing Rising International Software Co. Ltd., said hackers were increasingly getting used to attacking domain name servers of major websites because they were a chink in cyber security systems.

"Many websites like Baidu have almost perfect inner security system, but their DNS security is up to domain name registers," Wang said.

"DNS companies should pay more attention to security," Wang said.

Wu Xu, a graduate student in Beijing, said she tried to log onto Baidu.com at 9 a.m, only to find it was inaccessible.

"Then I turned to Google.com," she said, adding that Baidu.com had always been her first choice of Internet search engines.

"It is rare for Baidu.com to be down for so long as the company boasts high security protection," said Li Tiejun, an IT security engineer of Beijing Kingsoft.

The damaged site included a message in red saying, "This site has been hacked by Iranian Cyber Army." Some netizens said in posts that the hackers who called themselves "Iranian Cyber Army" had changed Baidu's DNS records and redirected traffic to another website.

But Wang said he had no information about the "Iranian Cyber Army".

The "Iranian Cyber Army" apparently also took down Twitter.com last month, posting a message similar to that on Baidu.com.

The company was still investigating the case, according to the statement. In another statement to Xinhua later in the afternoon, it called on DNS companies to resolve technical problems in DNS security.

The search engine, which claims 70 percent of China's Internet search market, had only been down only once previously, for half an hour in December 2006.