(CNN) -- The whistleblower website WikiLeaks said Sunday that it was under cyber attack, preventing it from posting tens of thousands of classified U.S. diplomatic cables, it said via Twitter Sunday.

Despite the glitch, five international news outlets which had obtained the documents ahead of time published details of the leaked documents on their websites.

The announcement of the apparent attack came shortly after the United States warned WikiLeaks editor-in-chief Julian Assange that publishing the papers would be illegal and would endanger peoples' lives.

The New York Times, The Guardian newspaper in England, and newspapers and magazines in three other European nations published portions of the new classified material on Sunday.

The site, meanwhile, was experiencing a distributed denial of service (DDOS) attack, it said. That's an effort to make a website unavailable to users, normally by flooding it with requests for data.

As of 3 p.m. Sunday, the site was inaccessible.

The U.S. State Department's legal adviser said Saturday that if any materials in the posting of documents by the site were provided by government officials without proper authorization, "they were provided in violation of U.S. law and without regard for the grave consequences of this action."

WikiLeaks indicated last week that it was preparing to release a new batch of previously classified U.S. military documents.

"Next release is 7x the size of the Iraq War Logs," the group stated via Twitter Monday. "Intense pressure over it for months. Keep us strong."

State Department Legal Adviser Harold Hongju Koh told Assange he was responding to a letter about the newest leak.

Koh wrote that the department had spoken with representatives from The New York Times and The Guardian newspapers, and the German magazine Der Spiegel about 250,000 documents the whistleblower organization provided to them for publication.

WikiLeaks said Sunday it had also given documents to El Pais in Spain and Le Monde in France.

Koh described the distribution as the "illegal dissemination of classified documents" and said it would "place at risk the lives of countless individuals" -- criticisms that have been repeated by U.S. officials after past postings on the site.

The information blitz from WikiLeaks is expected to offer a glimpse into the worldwide communications of the State Department and its 297 embassies, consulates and missions through what are commonly referred to as "cables."

Koh wrote that releasing such documents could jeopardize relationships with allies, military actions and anti-terrorism operations.

CNN has not had advanced access to the documents, unlike some media organizations, because the company declined to sign a confidentiality agreement with WikiLeaks.

CNN Senior State Department Producer Elise Labott contributed to this report.