Assange claims he's being prosecuted for political reasons - Sweden strongly denies this allegation.
Assange's lawyers argue that sending him to Stockholm could land him in Guantanamo Bay or on death row in the U.S. Many legal experts say that's extremely unlikely.
Also, U.S. officials are trying to build a criminal case against WikiLeaks, which has published a trove of leaked diplomatic cables and secret U.S. military files on the wars in Iraq and Afghanistan.
However, it is not clear what, if any, charges U.S. authorities could bring against Assange, and unclear how he could be classed as an "unlawful enemy combatant," which could expose him to detention at Guantanamo Bay.
Assange, 39, was arrested last month on rape and molestation accusations stemming from encounters with two women during a trip to Sweden last summer. He appeared briefly in a London court Tuesday ahead of a full extradition hearing on Feb. 7 and 8.
The Associated Press contributed to this report.