US rights activists Friday condemned a lack of disclosure in the case against WikiLeaks suspect Bradley Manning, saying there was even less transparency than proceedings against the alleged September 11 attackers.

A coalition headed by the Center for Constitutional Rights filed a petition asking the US Army to order the judge in Manning's court martial to allow access to government papers, court orders and transcripts of proceedings, "none of which have been made public to date."

Manning, whose trial is scheduled to start on September 21, is accused of "aiding the enemy" and dozens of other charges over his alleged leaking of documents to the site -- a charge that carries a potential life sentence.

Manning allegedly leaked hundreds of thousands of military logs from Iraq and Afghanistan and US diplomatic cables to WikiLeaks between November 2009 and May 2010, when he served as a low-ranking intelligence analyst in Iraq.

But the lack of access to legal documents in Manning's case amounts to "denial of the public's First Amendment rights," and "is clearly erroneous and amounts to an usurpation of authority," the campaigners' petition said.

"The contrast with the degree of public access provided for in the military commissions under way at Guantanamo is striking," it said.

"Courtroom proceedings at Guantanamo are open to public observers and also available for live viewing domestically via closed circuit television.

"Transcripts of these courtroom proceedings are posted in a time frame comparable to that provided for high-profile criminal trials," it added.

Manning, 24, last month faced pre-trial hearings at Fort Meade military base in Maryland, near the US capital. Earlier proceedings against him at the same base in December 2011 were conducted "largely outside the public view," those who signed Friday's petition said.