WikiLeaks founder Julian Assange accused President Barack Obama of "national security extremism" on Tuesday after an American military judge convicted Bradley Manning over the biggest leak of classified documents in US history.

Praising Manning as "the most important journalistic source that the world has ever seen", Assange said the US soldier, who prosecutors said had supplied WikiLeaks with hundreds of thousands of classified US documents, did not receive a fair trial and called for the verdict to be overturned.

"The government kept Bradley Manning in a cage, stripped him naked and isolated him in order to break him, an act formally condemned by the United Nations Special Rapporteur for torture. This was never a fair trial,"

Assange said from inside the Ecuadorean Embassy in London, his home for more than a year.

Assange said WikiLeaks and Manning's own legal team would not rest until the judgement was overturned.

"It is a dangerous precedent and an example of national security extremism. It is a short-sighted judgment that cannot be tolerated and it must be reversed."

Assange did not confirm or deny whether Manning had in fact supplied WikiLeaks with classified documents, always using the word "alleged" when talking about the leak. WikiLeaks did not reveal its sources, he said, and always protected them.

Manning's conviction on five espionage and 14 other charges poses a potential problem for Assange since the severity of the judgment, which carries a jail sentence of up to 136 years, might deter would-be whistleblowers, the lifeblood of Assange's organisation.