A federal judge has ruled that Democrat Hillary Clinton must respond in writing in a lawsuit over her use of a private e-mail server while she was secretary of state.

In a two-page ruling issued on Friday, Judge Emmet Sullivan declined to order Clinton to testify in person in a case brought by the conservative group Judicial Watch.

The Democratic presidential candidate has 30 days to respond to Judicial Watch's questions over her use of an unauthorized server while she was the top US diplomat, the ruling reads.

In early July, US Attorney General Loretta Lynch said no charges would be brought against Clinton for using the private email server.

FBI Director James Comey however said that while there was no clear evidence that Clinton and her aides intended to violate secrecy laws, "there is information that they were extremely careless in their handling of very sensitive, highly-classified information."

Clinton has apologized for exclusively using the private email account and her own server during her time as secretary of state from 2009 to 2013.

Opponents argue that this breached rules about protecting classified documents from cyber attack and may have amounted to a crime.

The Judicial Watch lawsuit threatens to keep the e-mail controversy alive for the weeks leading up to the November 8 presidential election.