Washington: A Romanian hacker nicknamed "Guccifer" who helped expose the existence of private email server Hillary Clinton used when she was US secretary of state has been sentenced to more than four years in prison.

Marcel Lazar, 44, who used the alias online, had pleaded guilty in May to charges including unauthorised access to a protected computer and aggravated identity theft after being extradited from Romania.

Marcel Lazar, who used the alias Guccifer online, was jailed for hacking into Hillary Clinton's private email server. Credit:Screengrab/NBC

Lazar is believed to have hacked into email accounts of about 100 victims between 2012 and 2014.

They included celebrities, business executives and political figures such as Sidney Blumenthal, an adviser with whom Clinton corresponded using her personal email account; confidantes of former president George W. Bush; and former secretary of state Colin Powell.