US President-elect Donald Trump has said he believes Hillary Clinton is "guilty as hell" following the announcement the FBI is being investigated for its handing of the probe into Clinton's emails.

The US Justice Department yesterday (12 January) announced its plans to investigate the FBI over its handling of the probe into Hillary Clinton's use of a private email server in her role as secretary of state.

In a series of tweets, Trump said: "What are Hillary Clinton's people complaining about with respect to the F.B.I. Based on the information they had she should never have been allowed to run - guilty as hell.

"They were VERY nice to her. She lost because she campaigned in the wrong states - no enthusiasm!"

Trump has made his feelings on Clinton's email probe clear during the presidential election campaign, when he threatened to jail Clinton if he was elected, despite the FBI's investigation concluding the Democrat should not face criminal charges.

However, despite promising his voters he would ensure Clinton faced further investigation over the emails she sent using a private server in her role as secretary of state, he declined to do so following his election victory – prompting some of his fans to question how many other pledges he would backtrack on.

The investigation into the FBI's handling of the probe was announced by Inspector General Michael E Horowitz and will look into allegations FBI director James Comey did not follow department protocol in disclosing information about the investigation.