SHANGHAI - China's top search engine, Baidu Inc (BIDU.O), filed a lawsuit against its U.S.-based domain name service provider 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 Jan. 12, weeks after doing the same thing to popular microblogging site Twitter.

Baidu said it was seeking damages from, its United States service provider, 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 Inc, the domain name resolution of was unlawfully and maliciously altered, said Baidu in a statement emailed to Reuters.

Following the cyber attack, media carried snapshots of Baidu's home page showing a message This site has been hacked by Iranian Cyber Army against a dark background and the flag of Iran. [ID:nTOE60B05U]

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 hacking concerns [ID:nTOE60H01S].

Its Nasdaq-listed shares have gained around a fifth in value and touched a record $470.25 last Friday. (Reporting by Farah Master; Editing by Lucy Hornby and Jeremy Laurence)