The messages from the Government Communications Headquarters, obtained through an official request for information, included discussions by the government eavesdropping agency that speculated he was being framed by Swedish authorities seeking his extradition for questioning on rape allegations, The Guardian reported Sunday.
Assange spoke about the messages Sunday night during an interview with the Spanish television program, "Salvados."
The instant messages to which Assange had access remained unclassified by GCHQ.
Assange has spent the past 11 months in the Ecuadorian Embassy in London to avoid arrest and extradition to Sweden. He has maintained his innocence and said he feared he would be extradited to the United States. WikiLeaks posted millions of sensitive military and diplomatic documents and communiques
A GCHQ message from September 2012, read by Assange, allegedly said: "They are trying to arrest him on suspicion of XYZ. ... It is definitely a fit-up. ... Their timings are too convenient."
Assange provided little explanation about exactly who wrote the messages, The Guardian said.
Assange said GCHQ was unaware it may have anything on him that wasn't classified, The Guardian said.
"It won't hand over any of the classified information," he said. "But, much to its surprise, it has some unclassified information on us."
Another instant message conversation, as read by Assange, from August 2012 called the WikiLeaks founder a "fool" if he thought he could wait out Sweden's attempt to extradite him.
The agency confirmed it "responded formally to the subject who made the request. The disclosed material includes personal comments between some members of staff and do not reflect GCHQ's policies or views in any way."