A US military judge has refused to dismiss a charge that whistleblower Bradley Manning aided the enemy, including Osama bin Laden, by handing classified material to the anti-secrecy website WikiLeaks.

Col Denise Lind ruled out any possibility that the most serious charge facing private Manning at his court martial could be dropped, rejecting a motion put forward by the whistleblower's lawyer.

Lind's decision was backed by prosecution evidence that some information revealed to WikiLeaks by Manning was posted on the internet and reached Osama bin Laden.

Lind said Manning "was knowingly providing intelligence to the enemy" when he perpetrated the biggest leak in US history.

She also cited the "accused's training and experience and preparation" and the volume of classified information he disclosed to WikiLeaks.

Defence lawyer David Coombs argued that Manning did not have actual knowledge that by turning over the documents to WikiLeaks he was aiding al-Qaida. He said that Manning was guilty of negligence but not the "general evil intent" standard required to justify the charge.

The US government has always maintained that Manning should have known that the information could end up in the wrong hands, given the extensive training he underwent.

Possibility of life

The offence of aiding the enemy, which is just one of 21 counts facing Manning, carries the possibility of life in prison without parole.

Human rights activists and whistleblower advocates feared that leaking classified information could now be interpreted as assisting the enemy.

"The aiding-the-enemy charge is not only unconstitutional, it is unnecessary," said Ben Wizner, director of the American Civil Liberties Union's Speech, Privacy and Technology Project.

"The point of charging Manning in this way is to transform what was widely seen around the world as a valuable leak into treason. The government purports to criminalise any information that is published somewhere where the enemy can see it."

The judge also refused to dismiss a count of computer fraud.

Manning, a 25-year-old army intelligence analyst who served in Iraq, admitted in February to leaking hundreds of thousands of classified documents to WikiLeaks. But he denied that he was guilty of 12 counts, including aiding the enemy.

Washington has said that it will not pursue the death penalty against Manning.