The WikiLeaks website crashed in an apparent cyberattack after the accelerated publication of tens of thousands of once-secret State Department cables by the anti-secrecy organisation raised new concerns about the exposure of confidential US embassy sources.

"WikiLeaks.org is presently under attack," the group said on Twitter late Tuesday. One hour later, the site and the cables posted there were inaccessible. WikiLeaks updated its Twitter account to say that it was "still under a cyberattack" and directed followers to search for cables on a mirror site or a separate search system, cablegatesearch.net.

The apparent cyberattack comes after current and former American officials said the recently released cables and concerns over the protection of sources are creating a fresh source of diplomatic setbacks and embarrassment for the Obama administration.

It was not immediately clear who was behind the attack. The Associated Press reviewed more than 2,000 of the cables recently released by WikiLeaks. They contained the identities of more than 90 sources who had sought protection and whose names the cable authors had asked to protect.

Officials said the disclosure in the past week of more than 125,000 sensitive documents by WikiLeaks, far more than it had earlier published, further endangered informants and jeopardized US foreign policy goals.

The officials would not comment on the authenticity of the leaked documents but said the rate and method of the new releases, including about 50,000 in one day alone, presented new complications. "The United States strongly condemns any illegal disclosure of classified information," State Department spokeswoman Victoria Nuland said.

"In addition to damaging our diplomatic efforts, it puts individuals' security at risk, threatens our national security and undermines our effort to work with countries to solve shared problems. We remain concerned about these illegal disclosures and about concerns and risks to individuals.

"We continue to carefully monitor what becomes public and to take steps to mitigate the damage to national security and to assist those who may be harmed by these illegal disclosures to the extent that we can," she told reporters.

Neither Nuland nor other current officials would comment on specific information contained in the compromised documents or speculate as to whether any harm caused by the new releases would exceed that caused by the first series of leaks, which began in November and sent the administration into damage-control frenzy. WikiLeaks fired back at the criticism even as its website came under cyberattack. (AP)