The FBI has interviewed Democratic White House hopeful Hillary Clinton for more than three hours amid investigations into her use of a private email server while serving as US secretary of state.

"Secretary Clinton gave a voluntary interview this morning about her email arrangements while she was secretary," her spokesman Nick Merrill said in a statement on Saturday.

"She is pleased to have had the opportunity to assist the Department of Justice in bringing this review to a conclusion. Out of respect for the investigative process, she will not comment further on her interview,"

Clinton, who is considered the frontrunner to win the 2016 Democratic presidential nomination, has faced criticism for using a private email server to conduct government business.

The FBI is investigating Clinton's server to see if classified information was mishandled, a topic that has become a key issue in her presidential campaign.

Hillary Clinton and the tale of two phones

A State Department review released in May faulted Clinton for her exclusive use of the private email server during her tenure as secretary of state from 2009 to 2013, saying the practice posed a security risk and did not comply with government records laws.

Republicans have highlighted the issue to raise questions about Clinton's trustworthiness and judgment, while Clinton has expressed regret over the email controversy.

Saturday's interview came just one day after Attorney General Loretta Lynch sought to reassure the public that the investigation would not be tainted by political bias after she met in private with Clinton's husband, former president Bill Clinton, earlier this week.

Lynch said that she would follow the recommendations of prosecutors who are examining whether Hillary Clinton violated any laws through her use of the private email system.She said that she had made the decision several months ago, before criticisms arose surrounding her meeting with Clinton's husband.

While Clinton failed to cooperate with a State Department inspector general's audit of her email practices that concluded she had failed comply with agency standards on record keepin, the candidate's campaign said she was fully cooperating with the FBI.