The site appears to have recovered from the attack with the help of Amazon.com Inc.'s U.S.-based server-for-rent service. Late Tuesday morning, Web traffic to the site was handled by Amazon Web Services.
The site, which distributed a trove of U.S. diplomatic documents on Sunday, said in a Twitter message on Tuesday morning that it was under a "distributed denial of service attack," a method commonly used by hackers to slow down or bring down sites. WikiLeaks didn't identify the attackers.
The site, which is devoted to releasing anonymously submitted documents, also came under attack Sunday, but Tuesday's attack appeared to be more powerful.
Calls to Seattle-based Amazon.com were not immediately returned. Bahnhof, a Swedish Internet company that has been involved in hosting WikiLeaks, had no immediate comment on Tuesday.
In a typical denial-of-service attack, remote computers commandeered by rogue programs bombard a website with so many data packets that it becomes overwhelmed and unavailable to visitors. Pinpointing the culprits is difficult.
WikiLeaks said the malicious traffic was coming in at 10 gigabits per second on Tuesday, which would make it a relatively large effort. According to a study by Internet security company Arbor Networks, the average denial of service attack over the past year was 349 megabits per second, 28 times slower than the stream Wikileaks reported.
Sunday's attack didn't stop the publication of stories based on messages leaked from the U.S. State Department in several major newspapers. WikiLeaks had given the media outlets prior access to the diplomatic cables to publish in conjunction with their Sunday release on its site.
The cables, many of them classified, offer candid, sometimes unflattering assessments of foreign leaders, ranging from U.S. allies such as Germany and Italy to other nations like Libya, Iran and Afghanistan.