The Wikileaks website crashed on Tuesday night after an apparent cyber attack as the accelerated publication of tens of thousands of once-secret US diplomatic cables by the anti-secrecy organisation raised new concerns about the exposure of confidential American embassy sources.

" is presently under attack," the group declared on Twitter. An hour later, the website and the documents posted there were inaccessible. WikiLeaks later updated its Twitter account to say it was "still under a cyber attack"./ It directed followers to search for cables on a separate search system,

The apparent cyber attack comes after current and former US officials said the recently released files – and concerns about the protection of sources – are creating a fresh source of diplomatic setbacks and embarrassment for the Obama administration. It was not clear who was behind the attack on the WikiLeaks site.

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. 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 US strongly condemns any illegal disclosure of classified information," said a spokeswoman for the State Department, Victoria Nuland. "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."

WikiLeaks fired back at the criticism, however. "Dear governments, if you don't want your filth exposed, then stop acting like pigs," the group posted on Twitter.

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