China's top search engine, Baidu Inc, sued its U.S.-based domain name service provider Register.com Inc on Wednesday after a cyber attack interrupted its web services last week.

Hackers calling themselves the Iranian Cyber Army briefly hijacked Baidu's home page on January 12, weeks after doing the same thing to popular microblogging site Twitter.

In the lawsuit filed in Manhattan federal court in New York, Baidu said it sought damages under the Lanham Act for trademark infringement, breach of contract and negligence. Parts of the court document were redacted.

Baidu's account was vandalized so that Internet traffic intended for Baidu and accessed through the 'baidu.com' domain name was misdirected to a web page depicting an Iranian flag and a broken Star of David, the court document said. In addition, Baidu was locked out of its account -- thus precluding the re-routing of traffic to Baidu's website.

A spokeswoman for Register.com Inc in New York said the Baidu lawsuit was without merit.

Register.com takes cyber-terrorism very seriously and we are working closely with federal law enforcement officials who are investigating this crime as well as the recent similar attacks on Twitter and Google, spokeswoman Alice McGillion said in an emailed statement.

Baidu said that it was seeking damages from Register.com after web users around the world were unable to access its search engine for at least four hours.

As a result of the gross negligence of Register.com Inc, the domain name resolution of www.baidu.com was unlawfully and maliciously altered, Baidu said in a statement emailed to Reuters.

Baidu suffered serious injury to its reputation and business, including millions of dollars of lost revenue and out-of-pocket costs to restore service, the lawsuit said.

Following the cyber attack, news reports carried snapshots of Baidu's home page showing a message This site has been hacked by Iranian Cyber Army.

Outraged Chinese hackers retaliated by attacking Iran-based websites the following day.

Baidu, which holds the greatest share of the Chinese online search market, vaulted into greater prominence after Google's shock announcement last week that it may leave China over censorship and hacking concerns.

Its Nasdaq-listed shares have gained around a fifth in value and touched a record $470.25 last Friday.

The case is Baidu Inc v Register.Com Inc, U.S. District Court for the Southern District of New York, No. 10-444.

(Reporting by Farah Master in Shanghai and Grant McCool in New York; Editing by Lucy Hornby, Tim Dobbyn and Matthew Lewis)