WikiLeaks released a mysterious encrypted file on Wednesday after telling its followers on Twitter to stand by for "an important announcement."
WikiLeaks did not identify the contents of the 571 megabyte file and it could not be opened without a decryption key, which the anti-secrecy website said would be released "at the appropriate moment."
In July of last year, WikiLeaks posted what it called an "insurance file," which was also encrypted.
According to press reports, the 1.4-gigabyte file was intended for public release in the event of something untoward happening to WikiLeaks founder Julian Assange.
Assange is currently fighting extradition from Britain to Sweden where he is wanted to face questioning over allegations of rape and sexual assault.
The release of the latest mystery file comes just days after WikiLeaks published more than 130,000 US diplomatic cables from what it says is a cache of more than 251,000 documents.
Without confirming the authenticity of the latest documents, State Department spokeswoman Victoria Nuland said the United States "strongly condemns any illegal disclosure of classified information.
"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," Nuland told reporters.
"We remain concerned about these illegal disclosures and about concerns and risks to individuals," she said.
The New York Times reported that the latest dump of 133,887 confidential and secret documents included many containing the names of sensitive sources who could be at risk of reprisals if they were known to be talking to US diplomats.
In a message on its Twitter page, however, WikiLeaks said it was "totally false" that any of its "sources have been exposed or will be exposed."
Explore further: Mysterious 'insurance file' posted on WikiLeaks