WikiLeaks aims to release documents revealing that Swedish Foreign Minister Carl Bildt is a US spy as part of a "smear campaign" to stop Sweden from extraditing founder Julian Assange to the US, a Swedish daily reported Wednesday.

The whistle-blower website has threatened in an internal memo to publish a so far unknown diplomatic cable "where Foreign Minister Carl Bildt is shown to have been an informant for the US since the 1970s," the Expressen tabloid reported, saying it had seen the WikiLeaks memo.

Bildt "will have to step down. This will be the end of his political career," an unnamed person with access to the unpublished diplomatic cable was quoted as saying.

Bildt himself reacted to the report on his official blog Wednesday challenging WikiLeaks to publish "this in their opinion damning report."

"When that happens, this part of their planned 'smear campaign' will quickly fall to shreds," he wrote.

Assange is currently in Britain fighting extradition to Sweden, where he is wanted for questioning on rape and sexual assault allegations. WikiLeaks has long expressed concern if he is sent to Sweden, Stockholm would quickly send him on to the US.

Washington is eager to lay hands on the WikiLeaks founder after the organization's publication of hundreds of thousands of classified US diplomatic files. According to Expressen, the group's "smear campaign" against Sweden would be aimed at blocking Assange's further extradition.

"Julian Assange will most probably be freed from the sex crime suspicions, because that is just a trap," the unnamed person with insight into WikiLeaks told Expressen.

"What Assange is afraid of is that he either will be forced to testify in the trial against the arrested soldier and suspected WikiLeaks source (for the leaked diplomatic cables), or that he himself will be arrested and handed over to a US court to be tried for espionage against the US," he added.

AFP