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The FBI is investigating "serious" claims that hackers in China breached e-mails of top US officials, Secretary of State Hillary Clinton has said.

On Wednesday, Google said a campaign to obtain passwords was aimed at monitoring e-mail, and originated in Jinan, a city in eastern China.

The Chinese government on Thursday denied it was involved.

US government agencies have said no official email accounts were hacked and have released no further information.

"These allegations are very serious. We take them seriously. We are looking into them," Secretary of State Clinton said.

In a statement on an on Wednesday, Google said its security was not breached but indicated hundreds of individuals' passwords were obtained through fraud.

Chinese political activists and officials in other Asian countries were also targeted, Google said.

Foreign Ministry spokesman Hong Lei said China was working to crack down on global computer hacking, but he did not say whether the government would investigate Google's report.

"Allegations that the Chinese government supports hacking activities are completely unfounded and made with ulterior motives," Mr Hong said, according to the Reuters news agency.

The e-mail scam uses a practice known as "spear phishing" in which specific e-mail users are tricked into divulging their login credentials to a web page that resembles Google's Gmail web service (or which appears related to the target's work) but is in fact run by hackers, according to a by Google.

Having obtained the user's e-mail login and password, the hackers then tell Gmail's service to forward incoming e-mail to another account set up by the hacker.

In Washington, the ³ÉÈË¿ìÊÖ's Adam Brookes says it is extremely difficult for analysts to determine whether governments or individuals are responsible for such attacks.

But the fact that the victims were people with access to sensitive, even secret information, raises the possibility that this was cyber espionage, not cyber crime, our correspondent says.