The WikiLeaks website has fallen victim to an apparent cyber attack after the accelerated publication of tens of thousands of state department cables by the anti-secrecy organisation raised fresh concerns about the exposure of confidential US embassy sources.

"WikiLeaks.org is presently under attack," the group said on Twitter late on Tuesday. One hour later, the site and the cables posted there were inaccessible.

WikiLeaks updated its Twitter account to say 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 cyber attack comes after current and former US 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.

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 jeopardised US foreign policy goals. The officials would not comment on the authenticity of the leaked documents but said the rate and method of the latest 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.

WikiLeaks hit back at the criticism even as its website came under attack.

"Dear governments, if you don't want your filth exposed, then stop acting like pigs. Simple," the group posted on Twitter.

Some officials noted that the first releases had been vetted by media organisations who redacted them to remove the names of contacts that could be endangered. The latest documents have not been vetted in the same way.

"It's picking at an existing wound. There is the potential for further injury," said PJ Crowley, the former assistant secretary of state for public affairs who resigned this year after criticising the military's treatment of the man suspected of leaking the cables to WikiLeaks.

"It does have the potential to create further risk for those individuals who have talked to US diplomats. It has the potential to hurt our diplomatic efforts and it once again puts careers at risk."

Crowley set up a crisis management team at the state department to deal with the matter and said officials at the time went through the entire collection of documents they believed had been leaked and warned as many named sources as possible, particularly in authoritarian countries, that their identities could be revealed. A handful of them were relocated, but Crowley said others may have been missed and some could not be contacted because the effort would have increased the potential for exposure.

The latest releases "could be used to intimidate activists in some of these autocratic countries," he said. He said he believed that "any autocratic security service worth its salt" probably already would have the complete unredacted archive of cables but added that the new WikiLeaks releases meant that any intelligence agency that did not "will have it in short order".

WikiLeaks insisted it was "totally false" that any WikiLeaks sources have been exposed and appeared to suggest the group itself was not even responsible for releasing unredacted cables.