A view of a courthouse during a sentencing hearing for Marcel Lehel Lazar, a hacker known as Guccifer, at the US federal courthouse on September 1, 2016 in Alexandria, Virginia

A Romanian hacker who went by the online moniker "Guccifer" got four years and four months in prison Monday for releasing private emails, medical data and financial information from senior political figures, including Bush family members.

US District Judge James Cacheris sentenced Marcel Lehel Lazar, 44, in Alexandria, Virginia, just outside the US capital Washington.

He pleaded guilty in May after being extradited from Romania, where he was serving a seven-year sentence for prior hacks.

Lazar had claimed to have hacked into the account of Democratic presidential nominee Hillary Clinton, herself under fire for her use of a private email server during her stint as secretary of state.

FBI chief James Comey denied Lazar had been successful, but he was in fact able to hack into the emails of longtime Clinton confidant Sidney Blumenthal.

In all, Lazar has admitted gaining unauthorized access to the personal email and social media accounts of about 100 Americans from at least October 2012 to January 2014, including a relative of former president George W. Bush.

He notably intercepted photographs Bush had sent to his sister showing several artworks he had painted, including a self-portrait of himself taking a shower.

Wearing a green prison jumpsuit, Lazar was emotionless as his sentence was pronounced.

"Send a message that computer hacking is a serious offence," prosecutor Maya Song had asked the judge just before sentencing.

"He has not at one time expressed sorrow."

Defense attorney Shannon Quill said Lazar would soon be sent to Romania to serve his sentence there before being extradited once more to the United States to serve his long here.

"Guccifer" was arrested in January 2014 in Romania, in collaboration with the United States.

In Romania, he was found guilty of hacking into the email accounts of a number of public figures, including the head of domestic intelligence.