A group pressing for more open government Tuesday flouted a military ban and released a secret recording of testimony by US Army private Bradley Manning, accused of leaking a mass of classified files.

It marks the first time since Manning was arrested in May 2010 that the world has heard his voice as he awaits trial for giving material on the Iraq and Afghanistan wars, as well as diplomatic cables, to the WikiLeaks website.

Reporters covering a series of pre-trial hearings being held at a military tribunal at Fort Meade in Maryland have been explicitly barred from making any video or audio recordings or from taking photographs of Manning.

"We hope this recording will shed light on one of the most secret court trials in recent history," the Freedom of Press Foundation (FPF) said in a statement, as it published the audio-file on its website.

"We wish to make sure that the voice of this generation's most prolific whistle-blower can be heard -- literally -- by the world."

The tape was made during a one-hour statement last month, when Manning in a firm and assured voice explained that he leaked some 70,000 confidential government files to start a "public debate."

When he deployed to Iraq he found himself alienated from his comrades and at odds with an army that "seemed not to value human life," he told the hearing.

The Pentagon has informed military judge Denise Lind, who is presiding over Manning's case, that there was "a violation of the rules for the court," a spokesman said in a statement sent to AFP.

"The US Army is currently reviewing the procedures set in place to safeguard the security and integrity of the legal proceedings and ensure PFC Manning receives a fair and impartial trial."

In the audio, Manning is heard telling the hearing "it burdens me emotionally" that he witnessed a video of US soldiers in Baghdad celebrating as they gunned down what turned out to be civilians including two journalists.

"The most alarming aspect of the video to me, however, was the seemingly delightful bloodlust they appeared to have," he said of the "Collateral Murder" video which sparked global outrage when it was released by WikiLeaks.

He compared the soldiers "to a child torturing ants with a magnifying glass."

The 25-year-old, who is being held in military custody pending trial, said he would plead guilty to 10 of the less serious of the 22 charges against him, but would deny aiding America's enemies, a crime which carries a life sentence.

The court has agreed to accept his plea on the lesser allegations -- under which Manning faces 20 years in military custody. But the prosecution still intends to pursue the 12 remaining charges.

"Extreme secrecy in our courts, just like in our government's policies and our politics, is an anathema to democracy," the FPF said.

"The courtrooms of America should be open to the public so they can see and hear what is being done in their name."

FPF co-founder Daniel Ellsberg, who 42 years ago leaked the Pentagon Papers to the New York Times and was kicked out of Manning's hearing in December by the military, praised whoever made the recording and sent it to FPF.

"I'm very glad whoever did it. I applaud their doing it, it was in the interests of openness, accountability and a fair trial," he told AFP, adding that so far no official transcripts of the proceedings had been publicly released.

But Ellsberg admitted that the military will now "take greater efforts to do what they shouldn't be doing at all and that is to prevent any further recordings being made."

It was important to get Manning's voice out to the public though, Ellsberg said, adding that the reason he pleaded guilty at his own trial was in exchange for being able to make a public statement outlining his motives.

"I don't know how Bradley is able to hear of all the support he is getting, or indeed any of the criticism. He's in prison. He has access at some time to TV but whether he sees news programs I'm not clear."