A US federal judge in Washington says he may order Democratic presidential front-runner Hillary Clinton to testify under oath about whether she used a private email server as US secretary of state to evade public records disclosures.

US District Court Judge Emmet Sullivan signed an order on Wednesday 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."

"In sum, the circumstances surrounding approval of Mrs. Clinton's use of clintonemail.com for official government business, as well as the manner in which it was operated, are issues that need to be explored," Sullivan said.

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

There have been at least three dozen civil lawsuits filed, including one by The Associated Press, over public records requests related to Clinton's time as secretary of state from 2009 to 2013.

The Federal Bureau of Investigation (FBI) and four other US intelligence agencies are also investigating whether sensitive information that flowed through Clinton's email server was mishandled.

The inspectors general at the State Department is separately investigating whether rules or laws were broken.

Clinton has come under fire for using a private email account and server at her home in New York state for official emails when she was America's top diplomat between 2009 and 2013.

Critics, including likely Republican presidential election rival Donald Trump, say she endangered government secrets and evaded transparency laws.

Judicial Watch brought a lawsuit against the State Department to gain access to records related to a Clinton aide's employment.

"This is a very great victory for transparency and, despite the best efforts of the Obama administration and the Clinton camp, it looks like we might finally get some answers under oath about the Clintons' illicit email system," said Tom Fitton, president of Judicial Watch.

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.