A US Army intelligence specialist accused of leaking government secrets spent his 24th birthday in court on Saturday as his lawyers argued his status as a gay soldier before the repeal of "don't ask, don't tell" played an important role in his actions.

Lawyers for Pfc Bradley Manning began laying out a defence to show that his struggles in an environment hostile to homosexuality contributed to mental and emotional problems that should have barred him from having access to sensitive material.

Manning is accused of leaking hundreds of thousands of sensitive items to the anti-secrecy group WikiLeaks, including Iraq and Afghanistan war logs, State Department cables and a military video of a 2007 American helicopter attack in Iraq that killed 11 men, including a Reuters news photographer and his driver.

Prosecutors at the pre-trial hearing in a small courtroom on an Army post outside Washington began trying to connect Manning to the publication of that material by WikiLeaks. On Saturday, they presented six of about 20 witnesses they plan to call during the hearing being held to determine whether Manning will be court-martialled on 22 counts, including aiding the enemy.

Testimony included the first references since the hearing began on Friday to Adrian Lamo, a former hacker to whom Manning allegedly confessed his ties to WikiLeaks. The basis for the charges Manning faces are transcripts of a series of online chats with Mr Lamo.

The Obama administration says the released information has threatened valuable military and diplomatic sources and strained America's relations with other governments. Manning's lawyers counter that much of the information that was classified by the Pentagon posed no risk.

Army criminal investigators described evidence they collected that links Manning to the WikiLeaks website's collection of US military and diplomatic secrets.

But among the first issues to arise on Saturday was whether Manning's sexual orientation is relevant to the case against him. The defence revealed that Manning had written to one of his supervisors in Baghdad before his arrest, saying he was suffering from gender-identity disorder. He included a picture of himself dressed as a woman and talked about how it was affecting his ability to do his job and even think clearly.

Major Matthew Kemkes, a defence lawyer, asked Special Agent Toni Graham, an Army criminal investigator, whether she had talked to people who believed Manning was gay or found evidence among his belongings relating to gender-identity disorder. The condition often is described as a mental diagnosis in which people believe they were born the wrong sex.

Ms Graham said such questions were irrelevant to the investigation. "We already knew before we arrived that Pfc Manning was a homosexual," she said.