China's top Internet search engine Baidu said Wednesday it had sued a US web firm after its site was hacked, a new salvo in a growing spat after Google's threat to quit the country because of cyberattacks.

Google, which trails Baidu in market share in China, said last week it may abandon its Chinese search engine and possibly leave the country altogether over what it called a "highly sophisticated" attack by China-based hackers and state censorship.

The announcement has thrown a spanner into already frayed Sino-US ties, with Washington calling for an explanation and Beijing defending its right to filter information available on the web and telling foreign firms to obey the law.

"Today Baidu filed a lawsuit against its domain name registration service provider Register.com Inc. in a US court in New York, seeking damages over the incident of Baidu's service interruption last week," the Chinese firm said.

Nasdaq-listed Baidu said its site went down for hours after the January 12 attack by a group identifying itself as the "Iranian Cyber Army", the name used by hackers who briefly shut down the Twitter microblogging site last month.

Baidu spokesman Victor Tseng said at the time that service had been interrupted "due to external manipulation of its DNS (domain name server) in the United States".

In its statement Wednesday, Baidu said the software behind its domain name "was unlawfully and maliciously altered" as a result of the "gross negligence" of Register.com.

This led to "users from many places around the world being unable to access the Baidu website for a number of hours and causing serious damages to Baidu".

Register.com is a leading domain registration service that manages more than 2.5 million domain names, according to the company's website. Officials there were not available for immediate comment.

A Baidu spokeswoman said the company would not provide any further information on the lawsuit.

Kaiser Kuo, a Beijing-based Internet consultant, said the timing of the suit would fuel speculation that it was in response to the Google ultimatum.

"The suspicion will be that Baidu wants it to appear that even as its American rival was under attack from sophisticated Chinese hackers, Baidu was also suffering attacks that, though perhaps not originating in the US, were at least made possible by the negligence of an American domain name registrar," he said.

Google has said the cyberattacks against it were likely aimed at gaining access to the Gmail accounts of Chinese human rights activists, but has said it does not believe that goal was achieved.

The company is checking whether any of its China staff helped hackers lead the attack, the Wall Street Journal reported Tuesday.

Google said Tuesday it had postponed the launch in China of two mobile phones featuring its Android operating system and developed in conjunction with Motorola and Samsung -- evidence of potential commercial fallout from the row.

The phones were to be used on China Unicom's network.

A Chinese commerce ministry spokesman said last week the row would not affect overall trade and economic relations between China and the United States.

In Washington, senior US diplomat Kurt Campbell said Tuesday that US and Chinese officials had held "multiple meetings" over the Google case, and plan to have more in the coming days.

"Google's China employees are very important to Google. We are going to have communication with the government... we want to have a positive outcome with the government," Google spokeswoman Marsha Wang told AFP on Wednesday.

But some observers have suggested Google's tough stance may also have some basis in its difficulties in penetrating the Chinese market.

"If Google was making billions and billions of dollars, I think this would be a different story," said Jeremy Goldkorn, editor of Danwei.org, a website focused on China's media industry that is itself blocked in the country.

Baidu had 58.4 percent of China's search engine market in the final quarter of last year, followed by 35.6 percent for Google China, according to Internet research firm Analysys International.