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 WikiLeaks' Swedish servers came under attack again Monday, the whistle-blowing website's Swedish hosting company said.

"We are investigating cause," Mikael Viborg, head of the Internet service provider PRQ, told AP.

WikiLeaks confirmed in a tweet it was having difficulty with its PRQ severs.

Mikael Viborg said the servers had become unresponsive, probably due a so-called denial of service attack aimed at WikiLeaks.

WikiLeaks is already fighting to stay on the Internet. It switched its domain to Switzerland because its original web address was shut down by a US provider, as it continues to release thousands of classified US diplomatic cables.

WikiLeaks was on Sunday redirected back to a server in Sweden. Denis Simonet, president of the Pirate Party of Switzerland, said that his group moved wikileaks.ch after the main server in France went offline.

A denial-of-service attack (DoS attack) is an attempt to make a computer resource unavailable to its intended users. However, WikiLeaks claims to have several backups, so even a successful attack on the PRQ might not necessarily slow or shut it down, AP said.

The PRQ service has previously been criticized for hosting a Chechen rebel site and the filesharing website The Pirate Bay.