Some testimony in the espionage trial of a US soldier who passed secret files to WikiLeaks will be held behind closed doors to safeguard classified information, a US judge ruled Tuesday.

Evidence from 24 witnesses, including some US ambassadors, will be given in closed session as requested by prosecutors in the court-martial of army private Bradley Manning, which is due to start June 3.

The decision by Colonel Denise Lind, the military judge presiding over the case, is sure to spark renewed criticism from civil liberties groups who say the Manning case has been shrouded in excessive secrecy.

And it also comes as President Barack Obama's administration has come under fire for its crackdown on whistleblowers and the press, including for seizing journalists' phone and email records as part of its probes into leaks.

Critics note that, since taking office, Obama has invoked the Espionage Act, passed in 1917 to punish those who aid US enemies, six times -- twice as many times as all previous presidents since the law was passed combined.

At the final pre-trial hearing before the trial begins next month, Lind said she weighed Manning's right to a public trial against government concerns that sensitive information could be inadvertently revealed.

She concluded the only way to protect information deemed vital to US national interests was to close the courtroom to the public while issuing a censored transcript of the proceedings at a later date.

The judge added that the criteria for closing the courtroom needed to be "narrowly tailored" and that she would closely monitor the issue.

Manning, 25, has admitted to passing on a trove of secret government documents and diplomatic cables to the WikiLeaks website and faces charges he aided Al-Qaeda through his document dump.

If convicted on all charges, Manning faces life in prison.

But on Tuesday, US prosecutors accepted the defendant's guilty plea to a lesser offense in one of 22 counts in the case.

The offense was related to a State Department cable from the US embassy in Reykjavik, Iceland, which Manning has admitted to leaking.

Prosecutor Major Ashden Fein did not explain why the government chose not to pursue the greater offense, which had alleged Manning violated federal computer fraud laws and abetted US adversaries.

The move scales back the potential maximum sentence Manning faces to 154 years from 162 years, officials said.

Manning is offering to plead guilty to 10 of the less serious charges against him, which could see him sentenced to 20 years.

US authorities have painted Manning's massive leak as endangering lives and undermining American diplomacy.

But Manning's defense lawyer has sought to portray his client as a truth-teller seeking to shed light on US policies and secret decision-making.

He also has suggested the leaks did not help US adversaries nor cause undue harm to American interests.

But the judge warned Tuesday that when lawyers discuss the leaked diplomatic cables she would not allow the court-martial proceedings to turn into a wide-ranging debate on US foreign policy.

She voiced concern the trial could "devolve into many trials regarding international politics in many regions of the world" and that argument on the contents of cables had to be "brief, limited and focused."

Part of the government's case against Manning asserts that Al-Qaeda's late former leader, Osama bin Laden, requested a deputy retrieve documents online that Manning had leaked to WikiLeaks.

A Navy SEAL who participated in the raid that killed Bin Laden at his Pakistani compound had been expected to testify in the trial.

But Tuesday's proceedings indicated that the commando's testimony may not be necessary and will be reflected in documents submitted by the prosecution, officials said.