FORT MEADE, Maryland — A US military judge ruled Thursday that WikiLeaks suspect Bradley Manning will not be allowed to cite evidence at his trial alleging he caused no serious harm to the United States when he released a massive trove of secret government documents.

In a major setback for Manning's defense, Judge Denise Lind said what happened after the classified files were disclosed is irrelevant as to whether the US soldier committed the crime of leaking sensitive information that "could" cause damage to America's national security.

What occurred afterward "was not knowable to the accused" prior to the leak and therefore not pertinent to determining his guilt or innocence, said Lind, reading out her decision at a pre-trial hearing at Fort Meade, Maryland, northeast of the US capital.

Lind said prospective jurors could be "confused" if the trial allowed for a discussion of potential harm caused by the leaks.

Manning's civilian lawyer David Coombs had told the court on Wednesday that barring such evidence would mean the defense would be "cut off at the knees."

Coombs has suggested that his client aimed to shed light on government secrecy while carefully selecting what documents to release to the WikiLeaks website to avoid damaging US national security.

The US government has backed off initial claims that the WikiLeaks disclosures -- the biggest breach of US intelligence in history -- caused grievous harm to American interests, and internal assessments have confirmed that, according to Coombs.

Manning, 24, a US Army private, faces a possible life sentence for "aiding the enemy" by releasing hundreds of thousands of classified diplomatic cables and military logs from Iraq and Afghanistan to WikiLeaks.

The judge left open the possibility that the defense could argue it needed to cite information related to the effect of the leaks when cross-examining prosecution witnesses. But she said she would take up their requests on a case-by-case basis.

The trial is tentatively due to begin later this year but could be pushed back to as late as February, the judge said.