A US military court of appeals on Wednesday dismissed a lawsuit filed by journalists and a rights group seeking greater openness in the upcoming trial of WikiLeaks source Bradley Manning.

The Center for Constitutional Rights filed the suit asking the court to compel the trial judge to expand access to court documents and transcripts, saying much of the pre-trial proceedings were carried out in secret.

The court rejected the suit by a 3-2 vote, arguing that because those filing the suit were not parties to the Manning trial, it could only rule on the scope of public access after the proceedings had ended and a sentence had been made.

Members of the press and public will therefore have no way to appeal possible decisions by the trial judge to bar them from the courtroom during the actual trial, the group said.

"Bradley Manning's trial will now take place under conditions where journalists and the public will be unable as a practical matter to follow what is going on in the courtroom," the group's attorney, Shayana Kadidal, said.

"That ensures that any verdict will be fundamentally unfair, and will generate needless appeals afterwards if he is convicted," she added.

Manning admitted in February to leaking a trove of confidential war logs and diplomatic cables to Julian Assange's anti-secrecy website and said he would plead guilty to 10 of the less serious charges against him, which could see him sentenced to 20 years in military custody.

The 25-year-old, who was working as a US Army intelligence analyst when he was arrested in Iraq in 2010, has denied the more serious charge of "aiding the enemy," which could carry a life sentence.

Manning is set to go before a full court martial on June 3, with the trial expected to last 12 weeks at Fort Meade, near Washington. Preliminary hearings are also scheduled to take place behind closed doors on May 7 and 8.