US government prosecutors are expected to interview Democratic presidential candidate Hillary Clinton and her former aides as part of the federal probe into her personal email server during her time as Secretary of State.

The federal investigation, which is being overseen by US Justice Department prosecutors and FBI agents, could still take many weeks, The Los Angeles Times reported on Sunday.

No dates have been set for questioning Clinton or the advisors, but a federal prosecutor has called their lawyers in recent weeks to alert them that he would be doing so soon, according to two people familiar with the probe.

The interviews by federal prosecutors and FBI agents will play a significant role in helping them better understand whether Clinton or her aides knowingly or negligently discussed classified government secrets over a non-secure email system when she was the head of the US State Department.

The expected interviews are also an indication that much of the investigators' background work – recovering deleted emails, understanding how the server operated and determining whether it was breached – is nearing completion.

“The interviews are critical to understand the volume of information they have accumulated,” said James McJunkin, former head of the FBI's Washington field office. “They are likely nearing the end of the investigation and the agents need to interview these people to put the information in context. They will then spend time aligning these statements with other information, emails, classified documents, etc., to determine whether there is a prosecutable case."

FBI Director James Comey has said he has been regularly briefed on the investigation, which is being overseen by prosecutors in the Justice Department’s national security division.

Many legal experts believe that Clinton faces little risk of being indicted for using the private email system to conduct official business during her time as Secretary of State.

However, the investigation will continue to hurt Clinton’s presidential campaign through most, if not all, of the remaining presidential primaries.

US presidential candidate Bernie Sanders says his overwhelming victory in the Saturday caucuses can help him win the support of elected Democratic Party officials over Clinton.

Sanders believes his recent string of victories shows he can not only win the Democratic nomination, but also succeed in the general presidential election, including against Republican Party frontrunner Donald Trump.