The attacks prompted the American domain name provider, EveryDNS, to withdraw service, The Associated Press said.
"Wikileaks.org has become the target of multiple distributed denial of service attacks. These attacks have, and future attacks would, threaten the stability of the EveryDNS.net infrastructure," EveryDNS said in a statement.
"WikiLeaks.org domain killed by US everydns.net after claimed mass attacks," the tweet said.
It was not clear where the alleged attacks were coming from. WikiLeaks can now be found at wikileaks.ch.
WikiLeaks has faced intense criticism in recent months after publishing confidential documents about the war in Afghanistan and U.S. relations with Pakistan.
Secretary of State Hillary Clinton said the leaks put "people's lives in danger."
Earlier this week, WikiLeaks' Swedish server host said it was hit by a cyber attack just before the site published thousands of classified U.S. diplomatic messages.