The US Department of State inspector-general has released a report, revealing that Democratic presidential candidate Hillary Clinton did not seek permission to use a private email server for official business while she was secretary of state.

The document added that if she had have asked permission, it would not have been granted, due to cyber-security fears.

Members of Clinton's team argued that other previous secretaries of state had also used personal email for official business.

After Clinton left her post, her family lawyers sifted through the server, and removed thousands of personal emails.

The remaining 30,000 were examined by the State Department, and hundreds were found to be classified. The rest were released as public records, but critics have alleged she used the private server to conceal facts from voters.