A jailed cyber-hacker has revealed how he allegedly accessed hidden messages on Hillary Clinton's private phone.

Marcel Lehel Lazar, a hacker from Romania who also goes by the name of "Guccifer" said he easily and repeatedly gained access to Clinton's server while she was serving as secretary of state, in 2013.

"It was like an open orchid on the internet," said Lazar, according to NBC News.

Lazar gave the first clues that Hillary Clinton conducted her work as US secretary of state through a private email server, leading to an official probe initiated by the FBI and an 11-hour public testimony by Clinton on the subject.

How he allegedly hacked Clinton's email

Lazar was recently extradited to the US for hacking the email accounts of high profile US Government officials, including former presidents George Bush and George W Bush, and former secretary of state Colin Powell.

From a jail cell in Virginia, US, where he is awaiting trial, Lazar told Fox News that he gained access to Clinton's server through the AOL account of Sidney Blumenthal, her long-time confidant.

Lazar extensively researched Blumenthal online and was able to guess his security question. Once inside the account, he was able to find the IP addresses for Clinton and Powell's emails through the incoming messages.