Bradley Manning, the US soldier who has spent 10 months in solitary confinement on suspicion of having transmitted a huge trove of state secrets to WikiLeaks, now faces a possible death penalty.

The intelligence specialist, who is being held in the maximum security jail on Quantico marine base in Virginia, has been handed 22 additional military charges as part of his court martial process.

They come on top of initial charges of having illegally obtained 150,000 secret US government cables and handing more than 50 of them to an unauthorised person that carried a possible sentence of up to 52 years in prison.

Manning's lawyer, David Coombs, said that the most serious of the new charges was the Article 104 offence of "aiding the enemy". The charge carries a potential death sentence.

The charge involves "giving intelligence to the enemy", which is defined as "organised opposing forces in time of war but also other hostile body that our forces may be opposing such as a rebellious mob or a band of renegades". Such an enemy could be civilian or military in nature.

The charge sheet, like the original set of accusations, contains no mention by name of the enemy to which the US military is referring.

It could be WikiLeaks itself, which the US secretary of state, Hillary Clinton, has accused of launching an "attack on America". Or it could be a reference to enemy forces in Afghanistan.

A report by NBC News said Pentagon officials emphasised that some WikiLeaks material contained names of informants and others working with US forces whose lives could have been put in danger.

According to Coombs, the 22 new charges were preferred by Manning's commanding officer after he made his own assessment of possible offences in the case. Under the court martial procedure, a provisional hearing, known as an Article 32, will be held in late May or early June when final charges to be laid against Manning will be decided. At that stage it will be known for certain whether the private faces a possible death sentence in the court martial itself.

Manning is accused of being the single source of many sensational WikiLeaks disclosures of US state secrets, some of which were published alongside the Guardian and other papers round the world. They include aerial footage of a US military attack on civilians in Iraq, Afghanistan war logs and thousands of US embassy cables.

He is being held in Quantico in conditions that have elicited protests from numerous organisations, including his own supporter networks and Amnesty International. The UN is investigating whether his treatment, which includes being held in his 6ft by 12ft cell for 23 hours a day, amounts to torture.

Manning is being kept on a "prevention of injury" watch which requires him to be held on his own and viewed every five minutes, despite prison psychiatrists' opinion that he is not a danger to himself.

David House, a researcher at MIT who is one of very few people to have visited Manning in prison, told the Firedoglake news website that the "aiding the enemy" charge was similar to Richard Nixon's heavy-handed treatment of Daniel Ellsberg, who leaked the Pentagon Papers. Nixon called Ellsberg "the most dangerous man in America" and said he was "providing aid and comfort to the enemy".

"Today we see the Obama administration continuing the legacy Nixon started by declaring whistleblowers as enemies of the state. It is a sad and dangerous day for transparency advocates everywhere," House said.