The organisation, which publishes confidential documents, has been in the cross-hairs of the US for years.
Last week, CIA Director Mike Pompeo described it as a "hostile" intelligence service abetted by Russia, and Mr Assange as a "fraud".
But the group was last year praised by President Donald Trump for its work during the election campaign.
The release of hacked emails belonging to a Hillary Clinton aide were a factor in her losing the election, the Democratic candidate later claimed.