The US Federal Bureau of Investigation (FBI) is likely to interview Democratic presidential candidate Hillary Clinton in the next few weeks about her use of a private email server while she was US secretary of state.

The FBI has already interviewed some of Clinton’s aides, including Huma Abedin, the vice chairwoman of Clinton's presidential campaign, CNN reported on Thursday.

Melanie Newman, a spokesperson for the US Justice Department, which oversees the FBI, declined to comment.

The nine-month FBI investigation into whether laws were broken as a result of the server kept in her New York state home has overshadowed Clinton's campaign.

More than 2,000 emails sent and received by Clinton while working as the top US diplomat between 2009 and 2013 include classified information, which the government bans from being handled outside secure, government-controlled channels.

On Wednesday, a US federal judge in Washington said he may order Clinton to testify under oath about whether she used a private email server.

US District Court Judge Emmet Sullivan signed an order granting a request from the conservative watchdog group Judicial Watch to question six current and former US State Department staffers about the creation and purpose of the private email server.

Based on what might be discovered from those interviews, which are to be conducted over the next eight weeks, Sullivan says in his order an out-of-court testimony from Clinton "may be necessary."

That raises the possibility that Clinton could be ordered to testify in the middle of the US presidential race.

Clinton has come under fire for using her private email account and server. Critics, including likely Republican presidential election rival Donald Trump, say she endangered government secrets and evaded transparency laws.

On Tuesday, Clinton lost to her rival Bernie Sanders in Indiana’s crucial primary. Sanders has said he is still fighting to win the Democratic Party’s nomination, though he acknowledged it will be an uphill task.

Sanders has been undaunted at the challenge of catching up to Clinton, even though it would require taking two-thirds of delegates in the remaining states.