The request came as Private Bradley Manning, 23, appeared at a military court for the first time.
He faces 22 charges of obtaining and distributing government secrets - which he allegedly leaked to anti-secrecy site Wikileaks.
The Article 32 hearing will determine whether Pte Manning is to stand trial.
The hearing offers the first opportunity for his defence team to present their case since he was arrested in Iraq in May 2010 and placed in military custody.
It is taking place under tight security at an army base at Fort Meade, Maryland.
As the hearing opened, Pte Manning's defence team asked for the investigating officer - equivalent to a judge in a civilian court - to withdraw from the case, the BBC's North America editor Mark Mardell reports from the base.
Pte Manning was reported to be sitting in the courtroom dressed in military khaki and wearing black-rimmed glasses.
During the Article 32 hearing, which is similar to a pre-trial hearing, both prosecuting and defence lawyers will make their initial cases and are permitted to cross-examine witnesses.
Proceedings are expected to last around five days, after which recommendations will be made to a military general, who will decide whether to proceed to a full trial, according to his lawyer, David Coombs.
The BBC's Paul Adams says the soldier's defence team is likely to argue that little harm came of the leaks, and that their release was in the greater public interest. » | Mark Mardell, BBC North America editor | Friday, December 16, 2011
THE GUARDIAN: Bradley Manning hearing: defence lawyer turns fire on military accusers: On first day of preliminary hearing, investigating officer rejects defence's demand that he recuse himself » | Ed Pilkington and Matt Williams in Fort Meade | Friday, December 16, 2011
