Bradley Manning has revealed to his court-martial at Fort Meade, Maryland, that he tried to leak US state secrets to the Washington Post, New York Times and Politico before he turned in frustration to the new anti-secrecy website WikiLeaks.

Manning, the US solider accused of the biggest leak of state secrets in US history, read out a 35-page statement to the court that contained new detail on how he came to download and then transmit a massive trove of secrets to WikiLeaks. It contains the bombshell disclosure that he wanted to go to mainstream American media but found them impenetrable.

While he was on leave from Iraq and staying in the Washington area in January 2010 he contacted the Washington Post and asked would it be interested in receiving information that he said would be "enormously important to the American people". He spoke to a woman who said she was a reporter but "she didn't seem to take me seriously".

The woman said, according to Manning's account, that the paper would only be interested subject to vetting by senior editors.

Despairing of that route, Manning turned to the New York Times. He called the public editor of the paper but only got voicemail.

He then tried other numbers on the paper but also got put through to voicemail, and though he left a message with his Skype contact details, nobody called him back. Manning added he had also contemplated going to the website Politico, but harsh weather prevented him.

In Manning's statement, he provides a wealth of information about how he systematically downloaded and transmitted confidential information to WikiLeaks. That included the video of an Apache helicopter attack in Iraq – the so-called collateral murder video – as well as war logs and hundreds of thousands of diplomatic cables.

Manning insisted that he believed that the cables would not harm US interests, though he suspected it would embarrass the US government by revealing behind the scenes deal-making.

He also gave insight into the ethical reasons that he had for making such an enormous breach of military orders. Referring to the war logs form Iraq and Afghanistan, he said he felt they would reveal the "true costs of war".

"I felt we were risking so much for people who seemed to be unwilling to cooperate with us leading to frustration and hostility on both sides. I began to get depressed about he situation we were mired in year after year.

"We were obsessed with capturing and killing human targets on lists and ignoring goals and missions. I believed if the public, particularly the American public, could see this it could spark a debate on the military and our foreign policy in general as it applied to Iraq and Afghanistan. It might cause society to reconsider the need to engage in counter terrorism while ignoring the human situation of the people we engaged with every day."

Manning also revealed that he had lengthy and prolonged discussions with a senior member of WikiLeaks codenamed Ox, whom he went on to name after an author of a book he had read in 2009: Nathaniel Frank. He said he assumed that "Nathaniel" was Julian Assange – whom Manning pronounced as Ass-angy.